NE555 false triggering

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NyLØDa

Joined Sep 14, 2018
5
Hi everyone. I am trying to build IR relay switch and i am running into some problems. I am using TSOP 1736 to turn on ne555 in monostable mode, but i often get some false triggering. I measured output of the IR sensor and it's perfectly stable. I assume problem comes from ne555. I tried everything i could think of, like adding capacitors to some pins of 555 but nothing helped. There's schematic link below. Everything is the same except i am using phone charger to power the circuit and I don't have 7805 regulator in my circuit. Sorry for the long post. I am hoping someone will help me to resolve this issue.

http://www.theorycircuit.com/wp-content/uploads/2016/03/IR-remote-control-switch.png
 

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dendad

Joined Feb 20, 2016
3,987
Does the LED flash?
You could try a filter between the IR receiver and the 555 as there could be random noise pulses there.
Just a small cap across R4 may help. Also, reduce R4 value too.
I always have 10uF tantalum caps and 100nF ceramics cross the power supply rails, as well as the electro.
 

DickCappels

Joined Aug 21, 2008
8,171
It may be interesting to note that the designer of the original circuit put 1,000 uf on the output of the LM7805. That's very unusual. That might indicated that something in the circuits is very sensitive to noise on the 5V power supply. Is the 5Vfrom your wall wart well decoupled? You might have to change power supplies to get this to work.

Another thing is that TSOP1736 IR receiver. The LED being detected should be driven by a 50% duty cycle square wave at the center frequency for the specific sensor being used. In the case of the TSOP1736 that frequency is 36.0 kHz.
upload_2018-10-31_12-29-58.png
I am surprised that this works on DC since I thought the amplifier stages inside these IR receivers are AC coupled.

The lack of an adequate signal may be causing the amplifier internal to the TSOP1736 to turn its automatic gain control to maximum gain, thereby making the the TSOP1736 much more sensitive to electrical noise. There does not appear to be an electrostatic shield on the receiver, such as that found on some others. Adding electrostatic shielding might also help, but I think modulating the LED at 36.0 kHz is likely to give you the greatest improvement.

Datasheet: dalincom.ru/datasheet/TSOP1736.pdf
 
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