555 Tester 230404

Thread Starter


Joined Feb 26, 2011
Hello AAC Forum,

Built a 555 tester from
Simple 555 Tester
555_tester_Electronics _hub.jpgApplied the schematic to a prototype board called Datak 12-607b in Nov 2021.

Have 555 Tester 211229.jpgused the tester several times over the years to test timer ICs before deploying in circuits.
Pulled the trusty timer today and when
a 555 was inserted and the circuit powered up both the LEDs came on an stayed on.
The expected result is the LEDs should oscilate.
Have tried four new 555 ICs with same result.
It looks the tester has failed.
Any idea about to troubleshoot the circuit?


Allen Pitts


Joined Mar 14, 2008
Any idea about to troubleshoot the circuit?
Sounds like it may be oscillating too fast to see the LED blinking.
Check the values of C2, and the two 10k resistors.

Also note the relative brightness of the LEDs with the 555, and without the 555 in the socket.
When in the socket, the lower LED should be noticeable brighter due to the ≈66% 555 output duty-cycle with those resistor values.
If not, check the connection between the LEDs and 555 pin 3, as an open there will leave both LEDs ON with relatively equal brightness.
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Joined Jan 23, 2018
Either it is oscillating to fast to follow OR the circuit to pin 3 is open. If both seem bright then it would be oscillating. That would probably be due to a lack of timing capacitor capacitance. So check for open connections due to damaged solder joints.
OR, a failed IC socket. That would be TI brand sockets from "back then."
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