help needed hooking up 4 lm3914

[davekra]

Hi,
I'm trying to use 4 lm3914 for a preasure gauge using an mpx4250a transducer.
I'd like to have the first ten led's setup for dot mode from a voltage of .2 to 1.7.
The remaining 30 led's would be setup for bar mode from a voltage of 1.7 to 3.2.
I'd also like to be able to adjust the brightness for the entire display.

I’ve been able to hookup the chips to give me the dot and bar that I want but I'm having trouble with the signal input to each chip.  I can’t figure out how to divide the output of the transducer and have a brightness control.  I’ve tried to figure out the resistor values but can’t seem to get the calculations to work. 

Would anyone be willing to help me figure out how to hook this up?  Both the configuration and some approximate values for things?  I can play with the values if I have the schematic, I just cant seem to put it all together.

Thanks,
davidk

 

[audioguru2]

Instead of dividing the output of the transducer to feed the cascaded LM3914's, usually the signal for the lower-level ones are amplified with an opamp for each one.

The brightness of the LEDs for an LM3914 is determined by its reference current. A current sink at the reference output pin for each LM3914 can be fed by a variable DC voltage from a brightness pot.

 

[davekra]

I'm sorry, "dividing the output of the transducer" may have been an incorrect phrase.  I'm passing the output directly to each chip and trying to set the Rlo and Rhi to give me what I want.

I re-read the datasheet aloud to my disinterested wife and realized I was skimming over some important information when I read it to myself. :-[
The datasheet said to ground pin 8 and place a resistor from pin 7 to ground. 

I've put together a crude drawing of the circuit as it stands now. 

I've got the thing working sort of.  I'm still trying to figure out the correct values for resistors R5 - R10 but it's displaying the way I want.
For the brightness can I just put in the circuit, in the box on my drawing, in place on pin 7 and 8?  What values could I start with for the resistors?  Is there a way to control the brightness for all chips with one adjustment?

Thanks allot for your patience and help.
davidk

 

[davekra]

I've been looking at resistor networks and it seems any thing over 2 or 3 resistors and it gets verrrry hard to calculate.  So how bout having separate pots for each voltage needed?  I modified my drawing showing 5 pots to set the hi and lo voltages for each chip.

Will the separate pots idea work?  How bout the brightness control?  Is there a way to control the brightness for all chips with one adjusment or will the circuit in the box work for each chip?

Thanks,
davidk

 

[davekra]

I think I got the pots setup correctly.  I found I needed to put one side to + and the other to - with the wiper going to the signal in of the chips.

Theres still the question of brightness control.  Can it be done with one adjustment?

Thanks,
davidk

 

[MP1]

The amount of resistance between pin 7 and pin 8 (which is grounded) determines the LED brightness on a LM3914. This is because you have a constant 1.25 volts reference between pins 7 and 8.

MP

 

[davekra]

Thanks MP,
Last nigt I found that if I put a 2K resistor and a 100K pot between pin 7 and ground I got brightness control with a full sweep of the pot. 
The circuit is turning out to be a lot simpler than I thought it would be.  No op amp needed and whatnot.

I still have the question about having a single brightness control for all chips.  I trid to tie all the pin 7's through one resistor but no dice.  I also tried to put a diode between each pin 7 and the resistor but again nothing.  I'd really like to have one adjustment for brightness, can anyone sugest a way of wiring it?

Thanks,
davidk

 

[davekra]

Ok, I tried using an LM324 as a pulse width modulator to control the voltage to the led's through a transistor.  This does work however as more led's light in bar mode the brightness changes.  Is this not a good way to control the brightness?  Why is the brightness going down as more led's light up?  With 30 led's on full it's drawing about 680ma.

Another question is can I hook two 5volt regulators in parallel?  Will this distribute the load to both regulators?

Thanks,
davidk

 

[binkabir]

i can answer the second question on parallel regulators: actually i can used them as you said but i will be wise if add  a voltage double after each regulator. but that i not a good design technique.  :-[
preferable you can use a power amplifier to increase the current gain and a buffer to isolate the LED from the circuit  ;D

 

[audioguru2]

I tried using an LM324 as a pulse width modulator to control the voltage to the led's through a transistor.

Hi David,
The LM3914 is designed to have the brightness of its LEDs controlled by the current from pin 7. Four LM3914's can have their brightness controlled by a single pot that adjusts the current of four current sinks, as attatched.

With 30 led's on full it's drawing about 680ma.

Another question is can I hook two 5volt regulators in parallel?  Will this distribute the load to both regulators?

A single 7805 regulator can supply at least 1A if it has an input voltage of about 10V and has a medium heatsink. Why have 2 regulators?

Voltage regulators have slightly different output voltages. If they are in parallel then the one with the highest voltage will handle the entire load until it overheats or limits its output current. Then its output voltage falls down to the voltage of the other regulator who begins to supply current. When a regulator current limits then it is not regulating.

The regulator's datasheet shows a PNP power transistor used as a current booster for a regulator and a 2nd transistor to limit its short circuit current. ;D
View attachment 37410

View attachment 37411

 

[davekra]

Thanks Audioguru,
I put it together like your drawing and it works great. 

As far as the regulators, I was just trying to keep the thing as cool as possible.  It's going into a space without much air movement.  I'll be putting a heat sink on it when it's done.

Now I just have to put it into a nice box and install it.

Thanks all for your help.
davidk