# Random Flashing LED Schematic Circuit Diagram

In recent years, the chapter ‘flashing lights’ in its many incarnations, has already received plenty of attention in Elektor Electronics. Therefore, a newly presented flasher circuit has to have at least one special character in order to be considered for publication.

The version described here is therefore definitely not an ‘ordinary’ flasher. Unlike most other circuits, the on/off rhythm of this circuit is not regular, but random. The circuit will undoubtedly find applications in various games, while it may also be very appropriate as a ‘pseudo-alarm-indicator’ to deter potential burglars.

Obviously, a random flasher will require a little more circuitry than a standard version. As is shown in the schematic, Schmitt-trigger IC3a is used to build a conventional **oscillator**, which runs at a relatively low **frequency**. This signal is used to clock a shift **register** IC. By feeding back the various outputs of the shift register through three inverting **XOR gates** (IC2a/b/c), the level changes at the output QH of the shift register will exhibit a quasi-random characteristic. This voltage is applied to a high-efficiency LED (D1), which completes the flasher.

The circuit has been designed for a power supply voltage of 5 V. The current consumption is about 8 mA when the LED is on.

**a one-bit adder that adds any two bits together to output one bit**. For example, if we add 1 plus 1 in binary, we expect a two-bit answer, 10 (i.e. 2 in decimal). Since the trailing sum bit in this output is achieved with XOR, the preceding carry bit is calculated with an AND gate.

**Q = ( A + B ) (A + B)**

**= (A’ + B’) (A + B)**This equation looks like it can be implemented using NOR Gates. We need a total of five NOR gates (two for inverting A and B, one for NOR of A and B, one for NOR of A’ and B’, and the final one to obtain the above equation). The exclusive-or (XOR) function is a nonlinear function that returns 0 when its two binary inputs are both 0 or both 1. It returns 1 when its binary inputs are different.

**The XOR cannot be represented by a linear network or a two-layer network**.