A very simple 555 timer tester circuit is designed using 555 timer integrated circuit. With a 555 timer circuit connected in the slot we can detect if timer is working properly . The 555 timer tester circuit above is basically a led flasher circuit but with the 555 removed.
When you have a good 555 under test, the hi and lo leds are flashing steadily. If you have a defective 555 timer you may get both leds off, one or both on steady, or one or both on faintly. In all these case the 555 is defective.
Do Not insert a 555 (or take it out) with the power on. The flash-rate can be changed with different values for C1 and R2. Try it, its fun. At pin 5 there is a small ceramic 10nF capacitor. It is just there to filter out any noise and is optional.
Experiment with leaving C2 out. C2 can be a value of 0.001 to 0.01μF, but the latter is the most common value (555 documentation specifies to use a general purpose ceramic 0.01μF capacitor).
R3 and R4 resistors value can be from 100 ohms ,up to 330 ohms , depending on LEDs used .
This project must be powered from a 9 volts DC power supply , but a 9 volt battery is great for this project .