A very simple tachometer circuit can be designed using a 555 timer, like in this circuit diagram . An electrical signal taken from the low tension side of the distributor is converted into a voltage proportional to engine rpm and this voltage is displayed on a meter calibrated accordingly .
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.