Toggle switch using 555 problem

Thread Starter

irmanao

Joined Apr 29, 2017
87
I built a toggle switch using a 555 and it works great when it is powered by my bench power supply but acts up when powered by the 78l12. The output
sometimes goes high momentarily and then low. (The voltage on the pictures should be 12 V)





Any ideas??

thanks
 

AlbertHall

Joined Jun 4, 2014
11,395
Do you have a 100nF across the '555 power pins close to the chip?
Is anything else being powered from the 12V?
Also it is recommended that you connect a 100nF from CV to ground.
 

Thread Starter

irmanao

Joined Apr 29, 2017
87
I don't have the 100nF on the power pins, i'll try that. There is also a 12V relay to control a lamp through a transistor.
 

Thread Starter

irmanao

Joined Apr 29, 2017
87
Ok i added the capacitors but to no avail. When i connected the 100nF cap across the 555 the output never latched, it only went high momentarily, as opposed to without the cap when it sometimes did that. What could i be doing wrong, this should be easier?
 

AlbertHall

Joined Jun 4, 2014
11,395
This may be because the relay contact bounces generating multiple trigger pulses. Try the following value changes which increase the time constants in the circuit to be definitely longer than the contact bounce period.
R1 & R5 => 100k
R2 => 470k
 

Thread Starter

irmanao

Joined Apr 29, 2017
87
Nothing changed, in fact it now never latches. So why would the circuit be working from the bench power supply??
 
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