Dont understand Schematic

[Darkside]

I dont follow this schematic. I have looked and looked and its just not popping. I dont know what its refering to for "Source" and "Sink". the PDF can be found at http://www.web-ee.com/Schematics/Doorknob_Alarm/5vmom1.pdf

If anyone can shed some light on this please I have all the parts just I do not want to start until I know what that Source and Sink are for. Thanks.

 

[ante1]

Hi Darkside,

Welcome to this forum.

The source output is when you want to feed something with 5 Volts and the sink is if the output is suppose to pull to ground.

 

[Darkside]

The source output is when you want to feed something with 5 Volts and the sink is if the output is suppose to pull to ground.

So. If I understand correct if I wanted to hook this up to a Relay I would connect the pos lead to the Source and the Sink to the Ground of the Relay coil?

I am still kinda confused on it.

 

[audioguru2]

Hi Darkside,
Ante is correct, but it doesn't have much power to drive anything but a logic gate, due to its small output transistors and 47 ohm resistors. What are you going to drive with it?

Additionally, the outputs operate normally open at rest, and activate when you touch the input. If they are used to drive a logic input, the source output should have a pull-down resistor added and the sink output should have a pull-up resistor.

 

[Darkside]

Hi Darkside,
Ante is correct, but it doesn't have much power to drive anything but a logic gate, due to its small output transistors and 47 ohm resistors. What are you going to drive with it?

Additionally, the outputs operate normally open at rest, and activate when you touch the input. If they are used to drive a logic input, the source output should have a pull-down resistor added and the sink output should have a pull-up resistor.

I will likely going to use it as a general toggle. Like say use a MPS2222 Transistor to make act as a simple connection for a PC Reset button. nothing major I was just using the Relay as a example. I am just wanting to make a simple touch switch that will operate at 5v. I can and have made a 12v touch switch using a LM555 timer IC which worked GREAT at 12v but had issues at 5V.

 

[audioguru2]

Hi,
Do you know about using a resistor in series with the transistor's base to limit the current? And a resistor to ground to turn-off the transistor?

 

[Darkside]

Hi,
Do you know about using a resistor in series with the transistor's base to limit the current? And a resistor to ground to turn-off the transistor?

I know you can use a resistor to limit the current but if you mean for a specific reason then no. and The resistor to ground to turn off the Transistor? not sure. sorry for my noobish line of questions. I know alot of basics but some stuff is new to me like using mosfets.

 

[audioguru2]

Hi Darkside,
If you use the "source" output of the circuit to turn-on a 2N2222, its 47 ohm resistor will limit the transistor's base current to about 90mA which is much too high. Add a resistor of 1K to 10K in series with the base to limit the current to a reasonable amount.

The source output turns-on the transistor, but nothing much turns it off and certainly not quickly. If the transistor got hot, it would leak and may not ever turn-off. Add another resistor the same value as above from its base to its emitter.

 

[Darkside]

I see. so basically I need to just add say a 1K resistor in series to the 47ohm and then cross Base to the Emitter with another 1K and that should be fine. sounds ok to me. I am going to use it to turn on PC with the 2222. I may even use a 4n25 opto to isolate the motherboard from the circuit just incase.

 

[audioguru2]

That's correct with the 2N2222 connection.
The "sink" output by itself may be able to turn-on the PC, if it needs a logic low and has its own pullup resistor.

If you use an opto, it will also need a current-limiting resistor (for its LED), but a value probably lower than 1K since it doesn't have much gain. Check its datasheet with your load current.

 

[Kevin Weddle]

I think I recognize this circuit. Is this a pushbutton activated oscillator output? Like a telephone transmitter.

 

[audioguru2]

Hi Kevin,
"You don't understand the schematic". ???
It doesn't have an oscillator output. The entire circuit is simply a touch-activated momentary switch.
It is unique in that you don't have to touch two touch-pads at the same time to conductively activate it like most touch-switch circuits. Instead, it applies a 20KHz signal to the single touch-button and uses your body's capacitance to ground (10pF to 300pF adjustable) to turn-on the PN2907 transistor and diode. The resulting 20KHz pulses are filtered twice into smooth DC and amplified by the Schmitt trigger inverters that drive the output Mosfets. It will probably work even if you are wearing gloves. Notice that its ground requires a connection to earth ground.

What is a "telephone transmitter"? The microphone in a telephone's handset is usually called its transmitter.
Do you mean touch-tone (also called DTMF) signalling? It is usually produced by an IC with X-Y keypad inputs that has its frequencies accurately controlled by a crystal oscillator that it divides down.

 

[MP1]

It is unique in that you don't have to touch two touch-pads at the same time to conductively activate it like most touch-switch circuits.

There is nothing "unique" about the single pad touch switch. It has been out there for a long time. I recall seeing them in electronic music instrument kits as far back as the 70s. The two-pad touch switch just uses a different method of switching the circuit.

MP