General Purpose Flasher / Impulse Generator
There have been many published designs for light flashers, but we think you will agree ours is a little different! Not only is the flashing rate fully variable, but it is also possible to adjust the “mark to space ratio”, i.e. the “on” and “off” time of the lamps or LEDs switched by the output relay. The unit is therefore capable of a multitude of effects, from a sustained flash with a very short off period, to a brief strobe like effect which has a long period between flashes.
If the relay has changeover contacts, two sets of lights can be made to flash alternately, in the manner of train level crossing lights. It can also be used to switch motors or solenoids on and off periodically.
How it Works. IC1, a 555 timer, is configured as an astable multivibrator, producing a squarewave output. The frequency determining components are C2, R1 and VR2. The setting of VR1 determines the mark-to-space ratio of the squarewave output, which is used to pulse the relay on and off. Although the 555 is quite capable of driving a relay direct, connecting a relay coil across pin 3 and ground can have unpredictable results. A transistor driver is therefore included, comprising R2 and TR1. Diode D3 is necessary to remove inductive voltage spikes from across the relay coil, which if left unchecked could damage TR1.
Construction and Testing. There are no special precautions to observe concerning component layout. If you are a relative newcomer to soldering, however, take care not to overheat TR1, diodes D1-D3 and IC1. Use a socket to mount the latter if unsure of your soldering prowess! The circuit, once built and tested, does not require any setting up.