I'm currently finishing my GCSE engineering project and I am attempting to use a dual 555 timer circuit as a timer. I am using a schematic I copied off Kitronik Alarm Project Kit, but when simulating and after creating the physical circuit, it has failed to work. I have gone through everything I can think of to no avail.

I have researched how this circuit works, and understand simply how the capacitor charges and drains, but I still have very little understanding of how the circuit is meant to work, and I am having trouble diagnosing problems.

I have used crocodile clips to simulate the circuit, but no combination of reset/trigger switches results in anything changing in my eyes. I have consulted with teachers, but they have not been able to help. Here is an image of the simulated circuit:

Circuit no closed switches

Here is the original diagram I copied off, which can be found in this PDF: alarm_project_essentials_2_0

Alarm Schematics

Sorry if I have misused tags

  • \$\begingroup\$ You've mixed up pin 4 (reset) with pin 8 (Vcc). \$\endgroup\$
    – brhans
    Apr 30, 2019 at 14:14

2 Answers 2


There is an error copying it. It won't work if power supply to the NE555 chips come via a 10kohm resistor.

  • \$\begingroup\$ which resistor would that be? left 555 or right? \$\endgroup\$
    – J. Doe
    Apr 30, 2019 at 13:24
  • \$\begingroup\$ They both come through the same resistor. \$\endgroup\$
    – Dave Tweed
    Apr 30, 2019 at 13:26
  • \$\begingroup\$ Both. Why not open a 555 datasheet or Crocodile clips help file and double check it yourself which pin number has which functionality? It looks like you made one to one copy from the original diagram but the schematic symbols have very different pin mapping and you need to wire it accordingly. \$\endgroup\$
    – Justme
    Apr 30, 2019 at 13:30

Add the 330R resistor in series with the LED.

As the circuit stands the first 555 will trigger when you release the switch. To make it trigger when you push the switch swap the positions of the switch and 10k resistor.


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