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Missing Pulse Detector based on 555



Figure 1. Missing Pulse circuit using 555


The 555 in its mono stable mode is ideally suited to this application, and the CMOS 7555 equivalent mentioned earlier will offer low current consumption as well. A possible arrangement is shown in Fig.1 Transistor TR1 is a normal high-gain pnp type. This means that if the voltage on its base (b) is more positive than the voltage at its emitter (e) minus 0·7V, it will be turned off. In other words, for a 9V power supply, and with TR1’s emitter held at 9V via resistor R3 and potentiometer VR1 (capacitor C1 fully charged), TR1 will only turn on if its base is at 9V – 0·7V = 8·3V approximately. If the base voltage is greater than 8·3V, TR1 will be turned off. In Fig.1, TR1’s base is normally held at 9V via resistors R1 and R2, and so it is turned off. If the signal input is briefly connected to 0V, the transistor will turn on and discharge capacitor C1. The 555 will also be triggered and will start the timing cycle. Assuming that the input returns high almost immediately following it being taken low (i.e. just a short trigger pulse), the timing cycle will finish at the end of the set period, unless further 0V pulses are received at the input. Hence the output will remain high, indicating that “all is well”, but if the pulses stop, the output will go low. The values of capacitor C1, resistor R3 and potentiometer VR1, set the timing period, after which the alarm is raised. The purpose of using a variable resistor is so that the timing period can be adjusted; you could use a single resistor in place of R3/VR1 if preferred, as in previous circuits. However, a 1M ohm pot or preset for VR1 will provide a wide range of timings. Resistor R3 is needed in case VR1 is reduced to zero resistance, a condition that would cause a short-circuit via TR1.