Photoresistor triggered solenoid, need help

[[email protected]]

I am digital electronics noob and it's been a long time since I've even
tinkered with this stuff. I am trying to design/build a simple circuit
to test a software application I am developing. I have a 12v solenoid
that I want to fire/push when I light up a certain area of the monitor.

I was hoping to use a photoresistor which I can tape to the monitor. I
have an assortment of 5 but the one I was trying to use has a
resistance of 800 ohms when the monitor is lit and 12k ohms when the
monitor is dark.

Can anybody suggest some approaches or even better design a simple
circuit that I could use to do what I described.

Summary of current parts, I can purchase additional ones if needed:
12V/3A power supply
CdS Photoresistor (800 in light, 12k in dark)
NPN transistors
741 Opamp chip
12V solenoid (30Watt power rating)

 

[Dan Hollands]

I am digital electronics noob and it's been a long time since I've even
tinkered with this stuff. I am trying to design/build a simple circuit
to test a software application I am developing. I have a 12v solenoid
that I want to fire/push when I light up a certain area of the monitor.

I was hoping to use a photoresistor which I can tape to the monitor. I
have an assortment of 5 but the one I was trying to use has a
resistance of 800 ohms when the monitor is lit and 12k ohms when the
monitor is dark.

Can anybody suggest some approaches or even better design a simple
circuit that I could use to do what I described.

Summary of current parts, I can purchase additional ones if needed:
12V/3A power supply
CdS Photoresistor (800 in light, 12k in dark)
NPN transistors
741 Opamp chip
12V solenoid (30Watt power rating)

The concept is to take the 741 and use a pair of resistors from 12V to
Ground to make are referance voltage which is connected to one input on the
741. Use the photoresistor and a fixed resistor to make voltage the goes
above the and below the referance voltage as the light is on and off.

The output of the op amp will go hi or lo as the light goes on and off. use
the op amp out to drive the transistor thru a resistor to the base, connect
the emitter to ground and the collector to the solenoid with the other end
of the solenoid to +12V. Put a diode across the solenoid with the cathode to
the +12 to prevent the inductive kick when from the solenoid when it is
turned off from destroying the transistor.

You might have to swap inputs to the 741 to make the solenoid On and Off
under the correct conditions.

Dan

--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com

 

[T. Wilkins]

I am digital electronics noob and it's been a long time since I've even
tinkered with this stuff. I am trying to design/build a simple circuit
to test a software application I am developing. I have a 12v solenoid
that I want to fire/push when I light up a certain area of the monitor.

I was hoping to use a photoresistor which I can tape to the monitor. I
have an assortment of 5 but the one I was trying to use has a
resistance of 800 ohms when the monitor is lit and 12k ohms when the
monitor is dark.

Can anybody suggest some approaches or even better design a simple
circuit that I could use to do what I described.

Summary of current parts, I can purchase additional ones if needed:
12V/3A power supply
CdS Photoresistor (800 in light, 12k in dark)
NPN transistors
741 Opamp chip
12V solenoid (30Watt power rating)

It can be done straight with one NPN transistor like the 2N3904: one side of
the photoresistor to the +12v, the other side to the base of the transistor,
then another 33o ohm resistor from the base to ground. The emitter also to
ground.
A load from the collector to +12v cannot drive more than a hundred milliamp
so you may need to add a more powerfull transistor at the output or a small
relay (don't forget the reverse diode in parallel with a coil to kill
reverse voltage spikes when transistor switches off...)

Tim

 

[scada]

The concept is to take the 741 and use a pair of resistors from 12V to
Ground to make are referance voltage which is connected to one input on the
741. Use the photoresistor and a fixed resistor to make voltage the goes
above the and below the referance voltage as the light is on and off.

The output of the op amp will go hi or lo as the light goes on and off. use
the op amp out to drive the transistor thru a resistor to the base, connect
the emitter to ground and the collector to the solenoid with the other end
of the solenoid to +12V. Put a diode across the solenoid with the cathode to
the +12 to prevent the inductive kick when from the solenoid when it is
turned off from destroying the transistor.

You might have to swap inputs to the 741 to make the solenoid On and Off
under the correct conditions.

Dan

--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com

When you say "Light up the Monitor" are you just sellecting an area? If so,
you can write code to the serial or parrellel port that will energize the
relay.

 

[[email protected]]

Wow, thanks for the quick replies. I'm going to give the suggestions a
try in the morning. In regards to writing output to the serial or
parellel port, the idea with this circuit is to test the software
timing externally without adding any new components to the actual
system.

 

[Fred Bloggs]

Summary of current parts, I can purchase additional ones if needed:
12V/3A power supply
CdS Photoresistor (800 in light, 12k in dark)
NPN transistors
741 Opamp chip
12V solenoid (30Watt power rating)

It is non-trivial driving that big solenoid with a 741 comparator. You
will need something like this:
View in a fixed-width font such as Courier.

..
.. .------------------------------+-------+-----+------.
.. (+)-----+------+----. | | | |
.. | | | | | | |
.. | |\|741 | | S | |
.. | | \ | | S [3.3,1/2W] |
.. | | >--------[470]-. | S | |
.. | | / | | | S | |
..12VDC | |/| [2.7K] .|>|-+ [39/5W] S | |
.. +| | | |914 | | | | |
.. === | | | |< | | | |
.. 100U | +-----+--|2N2907A | | | |
.. | | | |\ | +-|>|-' |
.. | | | | |/ | 3A,100V +|
.. | | | +-----|2N3055 | ===
.. | | | | |> | 10U
.. | | [1K] [2.2K] | |/ |
.. | | | | +-----| 2N3055 |
.. (-)-----+------+----+----------' | |> |
.. | [100] | |
.. | | | |
.. '------------------------------+-------+------------+
..
..
..
..
..
..

 

[Fred Bloggs]

It is non-trivial driving that big solenoid with a 741 comparator. You
will need something like this:
View in a fixed-width font such as Courier.

.
. .------------------------------+-------+-----+------.
. (+)-----+------+----. | | | |
. | | | | | | |
. | |\|741 | | S | |
. | | \ | | S [3.3,1/2W] |
. | | >--------[470]-. | S | |
. | | / | | | S | |
.12VDC | |/| [2.7K] .|>|-+ [39/5W] S | |
. +| | | |914 | | | | |
. === | | | |< | | | |
. 100U | +-----+--|2N2907A | | | |
. | | | |\ | +-|>|-' |
. | | | | |/ | 3A,100V +|
. | | | +-----|2N3055 | ===
. | | | | |> | 10U
. | | [1K] [2.2K] | |/ |
. | | | | +-----| 2N3055 |
. (-)-----+------+----+----------' | |> |
. | [100] | |
. | | | |
. '------------------------------+-------+------------+
.

If the CdS is located any distance from the main circuit then you will
want to use shielded twisted pair to connect to it. A simple threshold
detector that requires no adjustment would be something like this:
View in a fixed-width font such as Courier.

..
.. 12V
.. |
.. .------+----------.
.. | | |
.. [3.9K] [4.7K] |
.. | | .-[33K]------.
.. | | | | |
.. | | | |\| 741|
.. .-[12K]-+---------+-----|+\ |
.. . ---. | | | | >---+->
.. | | | |/ +--------|-/
.. ----- | TP .---+-----|2N2222A | |/|
.. /\ /\| XXXXXXXXXXXXXX | |> | |
.. ----- | -------------' | | | |
.. CdS | | SHLD | [100K] | [4.7K] |
.. '--- ' | | | | |
.. | | | | |
.. '---+-------+------+----------+
.. |
.. ---
.. ///
..
..

 

[Winfield Hill]

Fred Bloggs wrote...

It is non-trivial driving that big solenoid with a 741 comparator. You
will need something like this:

. .------------------------------+-------+-----+------.
. (+)-----+------+----. | | | |
. | | | | | | |
. | |\|741 | | S | |
. | | \ | | S [3.3,1/2W] |
. | | >--------[470]-. | S | |
. | | / | | | S | |
.12VDC | |/| [2.7K] .|>|-+ [39/5W] S | |
. +| | | |914 | | | | |
. === | | | |< | | | |
. 100U | +-----+--|2N2907A | | | |
. | | | |\ | +-|>|-' |
. | | | | |/ | 3A,100V +|
. | | | +-----|2N3055 | ===
. | | | | |> | 10U
. | | [1K] [2.2K] | |/ |
. | | | | +-----| 2N3055 |
. (-)-----+------+----+----------' | |> |
. | [100] | |
. | | | |
. '------------------------------+-------+------------+

With only a 12V supply, the 1n914 isn't needed.

If the CdS is located any distance from the main circuit then you will
want to use shielded twisted pair to connect to it. A simple threshold
detector that requires no adjustment would be something like this:
.
. 12V
. |
. .------+----------.
. | | |
. [3.9K] [4.7K] |
. | | .-[33K]------.
. | | | | |
. | | | |\| 741|
. .-[12K]-+---------+-----|+\ |
. . ---. | | | | >---+->
. | | | |/ +--------|-/
. ----- | TP .---+-----|2N2222A | |/|
. /\ /\| XXXXXXXXXXXXXX | |> | |
. ----- | -------------' | | | |
. CdS | | SHLD | [100K] | [4.7K] |
. '--- ' | | | | |
. | | | | |
. '---+-------+------+----------+
. |
. ---
. ///

At this point the 741 isn't doing much, how about designing
it out of the circuit?

 

[Fred Bloggs]

I am trying to design/build a simple circuit
to test a software application I am developing. I have a 12v solenoid
that I want to fire/push when I light up a certain area of the monitor.

I was hoping to use a photoresistor which I can tape to the monitor. I
have an assortment of 5 but the one I was trying to use has a
resistance of 800 ohms when the monitor is lit and 12k ohms when the
monitor is dark.

Can anybody suggest some approaches or even better design a simple
circuit that I could use to do what I described.

Summary of current parts, I can purchase additional ones if needed:
12V/3A power supply
CdS Photoresistor (800 in light, 12k in dark)
NPN transistors
741 Opamp chip
12V solenoid (30Watt power rating)

This one does a little better on pickup suppression:
View in a fixed-width font such as Courier.

..
.. 12V
.. |
.. .------+----------.
.. | | |
.. [3.9K] [4.7K] |
.. | | .-[33K]------.
.. | | | | |
.. | | | |\| 741|
.. .-[12K]---+---------+-----|+\ |
.. . ---. | | | | >---+->
.. | | | |/ +--------|-/
.. ----- | TP .---+-[10K]-|2N2222A | |/|
.. /\ /\| XXXXXXXXXX | |> | |
.. ----- | ---------' | | | |
.. CdS | | SHLD | [100K] | [4.7K] |
.. '--- ' | | | | |
.. | | | | |
.. '---+---------+------+----------+
.. |
.. ---
.. ///
..
..

The solenoid driver portion:
..
.. .------------------------------+-------+-----+------.
.. (+)-----+------+----. | | | |
.. | | | | | | |
.. | |\|741 | | S | |
.. | | \ | | S [3.3,1/2W] |
.. | | >--------[470]-. | S | |
.. | | / | | | S | |
..12VDC | |/| [2.7K] .|>|-+ [39/5W] S | |
.. +| | | |914 | | | | |
.. === | | | |< | | | |
.. 100U | +-----+--|2N2907A | | | |
.. | | | |\ | +-|>|-' |
.. | | | | |/ | 3A,100V +|
.. | | | +-----|2N3055 | ===
.. | | | | |> | 10U
.. | | [1K] [2.2K] | |/ |
.. | | | | +-----| 2N3055 |
.. (-)-----+------+----+----------' | |> |
.. | [100] | |
.. | | | |
.. '------------------------------+-------+------------+
..

 

[John Devereux]

The solenoid driver portion:
.
. .------------------------------+-------+-----+------.
. (+)-----+------+----. | | | |
. | | | | | | |
. | |\|741 | | S | |
. | | \ | | S [3.3,1/2W] |
. | | >--------[470]-. | S | |
. | | / | | | S | |
.12VDC | |/| [2.7K] .|>|-+ [39/5W] S | |
. +| | | |914 | | | | |
. === | | | |< | | | |
. 100U | +-----+--|2N2907A | | | |
. | | | |\ | +-|>|-' |
. | | | | |/ | 3A,100V +|
. | | | +-----|2N3055 | ===
. | | | | |> | 10U
. | | [1K] [2.2K] | |/ |
. | | | | +-----| 2N3055 |
. (-)-----+------+----+----------' | |> |
. | [100] | |
. | | | |
. '------------------------------+-------+------------+
.

What's with all the 2N3055s? Surely a power mosfet would be better -
you could drive it straight from the 741. Perhaps with a pulldown or
use a LM358 instead, to make sure it turns off.

 

[Fred Bloggs]

Fred Bloggs wrote...

Fred Bloggs wrote:

It is non-trivial driving that big solenoid with a 741 comparator. You
will need something like this:

. .------------------------------+-------+-----+------.
. (+)-----+------+----. | | | |
. | | | | | | |
. | |\|741 | | S | |
. | | \ | | S [3.3,1/2W] |
. | | >--------[470]-. | S | |
. | | / | | | S | |
.12VDC | |/| [2.7K] .|>|-+ [39/5W] S | |
. +| | | |914 | | | | |
. === | | | |< | | | |
. 100U | +-----+--|2N2907A | | | |
. | | | |\ | +-|>|-' |
. | | | | |/ | 3A,100V +|
. | | | +-----|2N3055 | ===
. | | | | |> | 10U
. | | [1K] [2.2K] | |/ |
. | | | | +-----| 2N3055 |
. (-)-----+------+----+----------' | |> |
. | [100] | |
. | | | |
. '------------------------------+-------+------------+

With only a 12V supply, the 1n914 isn't needed.

Ugh- you're such a stickler for economizing on parts, but you never know
what transistor he will eventually use.

If the CdS is located any distance from the main circuit then you will
want to use shielded twisted pair to connect to it. A simple threshold
detector that requires no adjustment would be something like this:
.
. 12V
. |
. .------+----------.
. | | |
. [3.9K] [4.7K] |
. | | .-[33K]------.
. | | | | |
. | | | |\| 741|
. .-[12K]-+---------+-----|+\ |
. . ---. | | | | >---+->
. | | | |/ +--------|-/
. ----- | TP .---+-----|2N2222A | |/|
. /\ /\| XXXXXXXXXXXXXX | |> | |
. ----- | -------------' | | | |
. CdS | | SHLD | [100K] | [4.7K] |
. '--- ' | | | | |
. | | | | |
. '---+-------+------+----------+
. |
. ---
. ///

At this point the 741 isn't doing much, how about designing
it out of the circuit?

I see what you mean- but just in case pickup proves to be a problem, the
741 leaves the option of unlimited filtering of the CdS line while
maintaining microsecond grade switch times into the driver, and besides
using 741's is fun.

 

[Fred Bloggs]

The solenoid driver portion:
.
. .------------------------------+-------+-----+------.
. (+)-----+------+----. | | | |
. | | | | | | |
. | |\|741 | | S | |
. | | \ | | S [3.3,1/2W] |
. | | >--------[470]-. | S | |
. | | / | | | S | |
.12VDC | |/| [2.7K] .|>|-+ [39/5W] S | |
. +| | | |914 | | | | |
. === | | | |< | | | |
. 100U | +-----+--|2N2907A | | | |
. | | | |\ | +-|>|-' |
. | | | | |/ | 3A,100V +|
. | | | +-----|2N3055 | ===
. | | | | |> | 10U
. | | [1K] [2.2K] | |/ |
. | | | | +-----| 2N3055 |
. (-)-----+------+----+----------' | |> |
. | [100] | |
. | | | |
. '------------------------------+-------+------------+
.

What's with all the 2N3055s? Surely a power mosfet would be better -
you could drive it straight from the 741. Perhaps with a pulldown or
use a LM358 instead, to make sure it turns off.

Geez- thanks for the inside scoop) I don't see power MOSFETs in the
OP's parts list lineup though....2N3055s are available from RS and the
overhead is only 10% of his solenoid dissipation. The only RS MOSFET,
the IRF510, is unsuitable and they are $2 each.

 

[T. Wilkins]

It can be done straight with one NPN transistor like the 2N3904: one side
of the photoresistor to the +12v, the other side to the base of the
transistor, then another 33o ohm resistor from the base to ground. The
emitter also to ground.
A load from the collector to +12v cannot drive more than a hundred
milliamp so you may need to add a more powerfull transistor at the output
or a small relay (don't forget the reverse diode in parallel with a coil
to kill reverse voltage spikes when transistor switches off...)

Tim

I come back with this ASCII schematic. This can do the job without any op
amps.

+12V o---------+-------+-----+
| | |
| | |
| | |
| C |
| O |
| I |
[Cds] L |
| | |
| | |
| | |
| +-|>|-'
| | 1N4001
| | D1
| |
|/ Q1
+-----| 2N3055
| |>
R1 [330] |
| |
gnd o----------+-------+


When the Cds is 12K, there is only 0.3V at the base of Q1 so it is off.
When Cds is 800 ohms or even a few kiloohms, the base is heavily driven,
enough for a 2N3055 to latch the solenoid.
The impedances are so low that, unless the distance between the Cds and the
transistor is very long, there is no need for shielded wires. Even in
extreme conditions (space radiation, nearby transmitter, ligthning, etc ,
a small 0.1uF capacitor can be added at the base to filter any interfere.

 

[Fred Bloggs]

It can be done straight with one NPN transistor like the 2N3904: one side
of the photoresistor to the +12v, the other side to the base of the
transistor, then another 33o ohm resistor from the base to ground. The
emitter also to ground.
A load from the collector to +12v cannot drive more than a hundred
milliamp so you may need to add a more powerfull transistor at the output
or a small relay (don't forget the reverse diode in parallel with a coil
to kill reverse voltage spikes when transistor switches off...)

Tim

I come back with this ASCII schematic. This can do the job without any op
amps.

+12V o---------+-------+-----+
| | |
| | |
| | |
| C |
| O |
| I |
[Cds] L |
| | |
| | |
| | |
| +-|>|-'
| | 1N4001
| | D1
| |
|/ Q1
+-----| 2N3055
| |>
R1 [330] |
| |
gnd o----------+-------+


When the Cds is 12K, there is only 0.3V at the base of Q1 so it is off.
When Cds is 800 ohms or even a few kiloohms, the base is heavily driven,
enough for a 2N3055 to latch the solenoid.
The impedances are so low that, unless the distance between the Cds and the
transistor is very long, there is no need for shielded wires. Even in
extreme conditions (space radiation, nearby transmitter, ligthning, etc ,
a small 0.1uF capacitor can be added at the base to filter any interfere.

10mA base drive for a '3055 2.5A load makes for an Ic/Ib=250, that's not
even on the charts. Your circuit will hang . Why don't *you* breadboard it?

 

[Jim Thompson]

I am digital electronics noob and it's been a long time since I've even
tinkered with this stuff. I am trying to design/build a simple circuit
to test a software application I am developing. I have a 12v solenoid
that I want to fire/push when I light up a certain area of the monitor.

I was hoping to use a photoresistor which I can tape to the monitor. I
have an assortment of 5 but the one I was trying to use has a
resistance of 800 ohms when the monitor is lit and 12k ohms when the
monitor is dark.

Can anybody suggest some approaches or even better design a simple
circuit that I could use to do what I described.

Summary of current parts, I can purchase additional ones if needed:
12V/3A power supply
CdS Photoresistor (800 in light, 12k in dark)
NPN transistors
741 Opamp chip
12V solenoid (30Watt power rating)

It can be done straight with one NPN transistor like the 2N3904: one side
of the photoresistor to the +12v, the other side to the base of the
transistor, then another 33o ohm resistor from the base to ground. The
emitter also to ground.
A load from the collector to +12v cannot drive more than a hundred
milliamp so you may need to add a more powerfull transistor at the output
or a small relay (don't forget the reverse diode in parallel with a coil
to kill reverse voltage spikes when transistor switches off...)

Tim

I come back with this ASCII schematic. This can do the job without any op
amps.

+12V o---------+-------+-----+
| | |
| | |
| | |
| C |
| O |
| I |
[Cds] L |
| | |
| | |
| | |
| +-|>|-'
| | 1N4001
| | D1
| |
|/ Q1
+-----| 2N3055
| |>
R1 [330] |
| |
gnd o----------+-------+


When the Cds is 12K, there is only 0.3V at the base of Q1 so it is off.
When Cds is 800 ohms or even a few kiloohms, the base is heavily driven,
enough for a 2N3055 to latch the solenoid.
The impedances are so low that, unless the distance between the Cds and the
transistor is very long, there is no need for shielded wires. Even in
extreme conditions (space radiation, nearby transmitter, ligthning, etc ,
a small 0.1uF capacitor can be added at the base to filter any interfere.

10mA base drive for a '3055 2.5A load makes for an Ic/Ib=250, that's not
even on the charts. Your circuit will hang . Why don't *you* breadboard it?

It's Aylward VOLTAGE bias ?

...Jim Thompson

 

[Winfield Hill]

Fred Bloggs wrote...

Fred Bloggs wrote...

Fred Bloggs wrote:


It is non-trivial driving that big solenoid with a 741 comparator.
You will need something like this:

. .------------------------------+-------+-----+------.
. (+)-----+------+----. | | | |
. | | | | | | |
. | |\|741 | | S | |
. | | \ | | S [3.3,1/2W] |
. | | >--------[470]-. | S | |
. | | / | | | S | |
.12VDC | |/| [2.7K] .|>|-+ [39/5W] S | |
. +| | | |914 | | | | |
. === | | | |< | | | |
. 100U | +-----+--|2N2907A | | | |
. | | | |\ | +-|>|-' |
. | | | | |/ | 3A,100V +|
. | | | +-----|2N3055 | ===
. | | | | |> | 10U
. | | [1K] [2.2K] | |/ |
. | | | | +-----| 2N3055 |
. (-)-----+------+----+----------' | |> |
. | [100] | |
. | | | |
. '------------------------------+-------+------------+

With only a 12V supply, the 1n914 isn't needed.

Ugh- you're such a stickler for economizing on parts, but you never
know what transistor he will eventually use.

Not really an issue. With a 12V supply the 2n2907 has only 3.25V on
its base, and there's no transistor that has reverse Veb breakdown
below, say 5V, so a diode isn't needed to "protect" the transistor.
In fact the supply voltage can be raised to 18.5V for 5V breakdowns.
I've personally never seen an NPN breakdown voltage below 6V.

 

[John Devereux]

I am trying to design/build a simple circuit

to test a software application I am developing. I have a 12v solenoid
that I want to fire/push when I light up a certain area of the monitor.
I was hoping to use a photoresistor which I can tape to the
monitor. I
have an assortment of 5 but the one I was trying to use has a
resistance of 800 ohms when the monitor is lit and 12k ohms when the
monitor is dark.
Can anybody suggest some approaches or even better design a simple
circuit that I could use to do what I described.
Summary of current parts, I can purchase additional ones if needed:
12V/3A power supply
CdS Photoresistor (800 in light, 12k in dark)
NPN transistors
741 Opamp chip
12V solenoid (30Watt power rating)

The solenoid driver portion:
.
. .------------------------------+-------+-----+------.
. (+)-----+------+----. | | | |
. | | | | | | |
. | |\|741 | | S | |
. | | \ | | S [3.3,1/2W] |
. | | >--------[470]-. | S | |
. | | / | | | S | |
.12VDC | |/| [2.7K] .|>|-+ [39/5W] S | |
. +| | | |914 | | | | |
. === | | | |< | | | |
. 100U | +-----+--|2N2907A | | | |
. | | | |\ | +-|>|-' |
. | | | | |/ | 3A,100V +|
. | | | +-----|2N3055 | ===
. | | | | |> | 10U
. | | [1K] [2.2K] | |/ |
. | | | | +-----| 2N3055 |
. (-)-----+------+----+----------' | |> |
. | [100] | |
. | | | |
. '------------------------------+-------+------------+
.

What's with all the 2N3055s? Surely a power mosfet would be better -
you could drive it straight from the 741. Perhaps with a pulldown or
use a LM358 instead, to make sure it turns off.

Geez- thanks for the inside scoop)

I just knew you'd appreciate my pearls of wisdom there

I don't see power MOSFETs in the OP's parts list lineup
though.

No reason to suspect his "NPN"s are 2N3055's either though, right? If
he is going to have to buy a suitable transistor anyway, perhaps there
are better choices.

...2N3055s are available from RS and the overhead is only 10% of his
solenoid dissipation. The only RS MOSFET, the IRF510, is unsuitable
and they are $2 each.

Hmm... why is it unsuitable (technically)? Seems like it should
work. How much do all those transistors add up to, plus that 5W
resistor if he can get it?

 

[T. Wilkins]

I am digital electronics noob and it's been a long time since I've even
tinkered with this stuff. I am trying to design/build a simple circuit
to test a software application I am developing. I have a 12v solenoid
that I want to fire/push when I light up a certain area of the monitor.

I was hoping to use a photoresistor which I can tape to the monitor. I
have an assortment of 5 but the one I was trying to use has a
resistance of 800 ohms when the monitor is lit and 12k ohms when the
monitor is dark.

Can anybody suggest some approaches or even better design a simple
circuit that I could use to do what I described.

Summary of current parts, I can purchase additional ones if needed:
12V/3A power supply
CdS Photoresistor (800 in light, 12k in dark)
NPN transistors
741 Opamp chip
12V solenoid (30Watt power rating)

It can be done straight with one NPN transistor like the 2N3904: one side
of the photoresistor to the +12v, the other side to the base of the
transistor, then another 33o ohm resistor from the base to ground. The
emitter also to ground.
A load from the collector to +12v cannot drive more than a hundred
milliamp so you may need to add a more powerfull transistor at the output
or a small relay (don't forget the reverse diode in parallel with a coil
to kill reverse voltage spikes when transistor switches off...)

Tim

I come back with this ASCII schematic. This can do the job without any op
amps.

+12V o---------+-------+-----+
| | |
| | |
| | |
| C |
| O |
| I |
[Cds] L |
| | |
| | |
| | |
| +-|>|-'
| | 1N4001
| | D1
| |
|/ Q1
+-----| 2N3055
| |>
R1 [330] |
| |
gnd o----------+-------+


When the Cds is 12K, there is only 0.3V at the base of Q1 so it is off.
When Cds is 800 ohms or even a few kiloohms, the base is heavily driven,
enough for a 2N3055 to latch the solenoid.
The impedances are so low that, unless the distance between the Cds and
the transistor is very long, there is no need for shielded wires. Even in
extreme conditions (space radiation, nearby transmitter, ligthning, etc
, a small 0.1uF capacitor can be added at the base to filter any
interfere.

10mA base drive for a '3055 2.5A load makes for an Ic/Ib=250, that's not
even on the charts. Your circuit will hang . Why don't *you* breadboard
it?

Not enough gain?
All right! Then let's go for a cheap Darlington...

+12V o---------+-------+-----+
| | |
| | |
| C |
| O |
| I |
| L |
[Cds] | |
| | |
| +-|>|-|
| |
| +--+
| | |
| |/ | Q2
+--| | 2N2904
| |> |
| | |/ Q1
+-| 2N3055
| |>
R1 [330] |
| |
gnd o----------+--------+

 

[T. Wilkins]

Not enough gain?
All right! Then let's go for a cheap Darlington...

+12V o---------+-------+-----+
| | |
| | |
| C |
| O |
| I |
| L |
[Cds] | |
| | |
| +-|>|-|
| |
| +--+
| | |
| |/ | Q2
+--| | 2N2904
| |> |
| | |/ Q1
+-| 2N3055
| |>
R1 [330] |
| |
gnd o----------+--------+

Oops again! Damn keyboard..!
I meant 2N3904 for Q2 of course, as the 2904 is a PNP... even worse lockup!

Tim

 

[John Devereux]

Not enough gain?
All right! Then let's go for a cheap Darlington...

+12V o---------+-------+-----+
| | |
| | |
| C |
| O |
| I |
| L |
[Cds] | |
| | |
| +-|>|-|
| |
| +--+
| | |
| |/ | Q2
+--| | 2N2904
| |> |
| | |/ Q1
+-| 2N3055
| |>
R1 [330] |
| |
gnd o----------+--------+

You need some hysteresis too, otherwise when the darlington is half-on
it will dissapate too much power (or oscillate, possibly).

You could switch a led too, then allow some of its light to shine on
the Cds!