Issue with 556 Latch circuit

Thread Starter

alwi

Joined Feb 12, 2023
3
Hi,
I am a bit of a newbie with electronics but I saw a circuit for a 555 LED latch and had a go at turning a schematic into a finished vero board project. This worked well, so having seen that it has a big brother, the 556 dual timer, I tried designing and implementing another simple cicuit. Now, the first project merely turns an LED on or off with a momentary switch. One press for on, one for off. The 556 dual circuit has two LEDs and switches - and yes, one switch for each LED. All fired up with my previous success, I used TinyCAD and VEECad to produce the stripboard design. Now we come to the issue.... Pressing each switch does indeed turn on and off the associated LED, but unfortunately also turns OFF the other LED if it happens to be on.

I have uploaded the schematic and board design (apologies for the amateurish efforts). I would be very grateful if someone could point out where I have gone wrong.

Notes: The LEDs and their resistors are off board at J7 and J8, and J9 and J10. Power at J5/J6. The momentary switches are across J1/J2 and J3/J4.

Thanks,
alwi
 

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Alec_t

Joined Sep 17, 2013
14,280
Welcome to AAC.
When a 555 (and I assume 556) output changes state there is a very brief supply current surge. I suspect that is causing your problem. Try connecting a 470uF decoupling cap directly between the supply pins of the IC.
 

crutschow

Joined Mar 14, 2008
34,285
Also connect two capacitors (10nF or greater), one from pin 3 to ground and one from pin 11 to ground.
Those filter the internal reference voltages which determine the trip points for the THRES and TRIG inputs.
 

Thread Starter

alwi

Joined Feb 12, 2023
3
Welcome to AAC.
When a 555 (and I assume 556) output changes state there is a very brief supply current surge. I suspect that is causing your problem. Try connecting a 470uF decoupling cap directly between the supply pins of the IC.
Hi,
I put a 100uF electrolytic (all I had) across J5 and J6 which did the trick. Now I know one thing that a decoupling/smoothing cap does.
Thanks,
alwi
 

crutschow

Joined Mar 14, 2008
34,285
Now I know one thing that a decoupling/smoothing cap does.
Yes, all electronic circuits need power supply decoupling to insure proper operation.
Most circuits should also have a 100nF ceramic cap across the power and ground to each IC to suppress any high frequency transients that the electrolytic doesn't, due to it's higher inductance and ESR.
 
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