Random running lamp circuit

If a number of the outputs of a shift register are fed back in a special style via an EXOR gate to data input, then the Q outputs of the register will run through the maximum potential number of respectively different logic states.

By using the Q outputs to drive LEDs a visual representation of the truth table for a shift register with EXOR feedback is obtained. At each clock pulse a logic “1” (LED lights up) or “0” moves up one place, so that the net result is running light.

In this circuit, the Q outputs of the shift register (IC2), which is seven bits long, are routed via an EXOR gate (N1 … N4) back to pin 7, the data input, of the register. The clock signal is provided by IC1. The clock frequency, and hence the speed of the running light, can be altered utilizing P1.

R4 … R17 and T1 … T17 are included to drive the LEDs. R18, R19, C3, and D8 ensure that when the circuit is switched on, a logic “1” appears at the data input, which means that the register output state can never be all zeroes. The circuit can be extended for use as a light organ. The LEDs are replaced by optocouplers which, via the necessary hardware (Triac, etc.) control incandescent lamps.

A rectified version of the audio signal is fed via R3 to the control input frequency of the register and hence the speed of the running light. Capacitor C2 may then be omitted. Varying the control voltage between 0 and 15V will result in the clock frequency being varied between 50 and 150% of the value obtained with the control input left floating.