One-shot circuit is programmable

J Jayapandian, IGCAR, Tamil Nadu, India -- EDN, 12/21/2000

Figure 1 shows how to digitally program the on-time of a one-shot multivibrator circuit. More and more, the Internet is playing a role in control operations in industrial and R&D endeavors and in household appliances. One-shot circuits are popular choices for the on/off control circuitry. You can interface the programmable one-shot design in Figure 1 with any intelligent system, such as a PC, a µP, or a µC. The design uses a low-cost NE555 timer and an 8-bit AD7524 D/A converter. The timer IC is connected in a one-shot configuration with an on-time transfer function of t=1.1 RC. The control voltage on Pin 5 of the 555 can change the threshold of the comparator in the timer IC, thereby changing the on-time of the one-shot's output. In other words, by selecting the voltage on this pin, you can control the pulse width of the output waveform of the timer. The 8-bit DAC, with its MC 3104 op-amp buffer, provides programmable control of the one-shot's pulse width. You can send the required 8-bit word to the DAC via a PC's parallel port or from a µP or µC. The maximum on-time of the one-shot is a function of the R and C values. You can vary the pulse width from minimum to maximum by changing the bit pattern at the DAC's input from 00 to FF.