Dual Op Amp false triggering on motion sensing circuit

Thread Starter

jaricurry

Joined Nov 9, 2018
7
Hello Everyone,

I am putting together a low voltage 3V circuit that senses motion via a PIR sensor, then the signal is amplified by a dual channel op amp that triggers a monostable vibrator IC to output a high voltage to a mosfet which will activate a LED light for a set time. The circuit works great on the breadboard... but on the PCB circuit the PIR and OP amp work fine but once I install the monostable vibrator the op amp keeps retriggering once the set time of the monostable vibrator kicks out. Any suggestions? Attached is the circuitcicuit.png
 

Picbuster

Joined Dec 2, 2013
1,027
Quick look : missing decoupling cap @ Ic's
Advice : please use the correct symbols allowing us to identify components

Picbuster
 

Thread Starter

jaricurry

Joined Nov 9, 2018
7
cicuit2.png
Quick look : missing decoupling cap @ Ic's
Advice : please use the correct symbols allowing us to identify components

Picbuster
Thx for Looking picbuster,

I added the circled in red decouplers and it is still false triggering but works great on the breadboard with the exact same components! Is that normal?

2) Which symbols do I have wrong?

Thx
 

Picbuster

Joined Dec 2, 2013
1,027
The opamp and u2 ( is this a fet or the PIR?)

If this is working on a breadboard and not on a PCB
you should check:
1: power supply ripple ( high output impedance could introduce this type of problems)
2: Layout of the PCB (in- and output tracks not close to each other)
Ground layers decoupling as close as possible to chip
I do see all very high rated resistors beautiful but also vulnerable to (weak) external signals.

Check all signals with a scope.

Picbuster
 

Thread Starter

jaricurry

Joined Nov 9, 2018
7
ok thank you. Now I understand what you mean using the correct symbols. I will change it. I had the decoupling capacitors away from the ICs so I changed that as well as added extra decoupling footprints to each IC and ordered some new PCBs to do some experimenting.
 

AlbertHall

Joined Jun 4, 2014
11,691
I don't know whether either of these observations has anything to do with your problem but:
+In_B has no dc path. I would be surprised if this didn't cause a problem.
Out_B has a 100nF directly to ground.
 

Thread Starter

jaricurry

Joined Nov 9, 2018
7
I don't know whether either of these observations has anything to do with your problem but:
+In_B has no dc path. I would be surprised if this didn't cause a problem.
Out_B has a 100nF directly to ground.
Hi Albert, the outB has the footprint to ground but I have not been using it.

+ln_B ... What do you mean by has no dc path?
 

AlbertHall

Joined Jun 4, 2014
11,691
Hi Albert, the outB has the footprint to ground but I have not been using it.

+ln_B ... What do you mean by has no dc path?
The only thing connected to +In_B is a capacitor. The input pin will have a DC bias current which will be very low but will not be zero. This current has nowhere to go other than to charge the capacitor. Therefore the voltage on that pin is undefined.
 

Thread Starter

jaricurry

Joined Nov 9, 2018
7
The only thing connected to +In_B is a capacitor. The input pin will have a DC bias current which will be very low but will not be zero. This current has nowhere to go other than to charge the capacitor. Therefore the voltage on that pin is undefined.
So you're saying it needs a direct path to the ground or to the V+?
 

AlbertHall

Joined Jun 4, 2014
11,691
Yes, and if I understand your circuit correctly it should have a resistor to V+ - leave the capacitor there to filter any supply noise.
 

sjsdorsay

Joined Dec 13, 2015
8
I agree with what AlbertHall is saying: that pin needs a DC path. The example circuit for your chosen amplifiers (TLV8542) show that you should have a DC bias to +In_B. It looks to me that they bias it at 50% of 3.3V.

upload_2018-11-13_10-6-32.png
 
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