It is the schematic diagram of a pulse electronic tachometer charged and discharged by a capacitor. When the engine is working, the contacts of the distributor are constantly opened and closed, and the opening and closing times are directly proportional to the engine speed, that is, the contacts are opened and closed twice for each revolution of the crankshaft of a four-stroke four-cylinder engine; The contacts of the six-cylinder machine are switched on and off for three times, and the intermittent current is generated by the switching on and off of the contacts. After being shaped by the integrating circuits R 1, R2, C 1, it is sent to the transistor VT 1, thus obtaining a rectangular current with a fixed amplitude (current value) and pulse width (time). This current passes through the milliammeter mA, and when the contacts are closed, the transistor VT/. The charging circuit of transistor VT 1 forms the path of the battery anode, resistor R3, capacitor C2, diode VD2 and battery cathode. When the contacts are separated, the base potential of the transistor VT 1 approaches the positive pole of the power supply, and VT 1 changes from off to on. At this time, the charge charged by the capacitor C2 is discharged by the milliammeter, and its discharge loop is the anode of the capacitor C2-transistor vt 1- milliammeter-diode VD2 and then returns to the cathode of C2. The contacts are repeatedly opened and closed, and the above process is repeated. Diode VD2 provides a charging circuit for container C2, and VD 1 provides a discharging circuit for C2. Since the charge and discharge quantity Q of C2 is directly proportional to the capacitor C and the voltage U across the capacitor, that is, Q=CU, let the average discharge current of each period (t) be: I = Q/t = Cu/t = CUF, where: F- is the opening and closing frequency of the contact. When the power supply voltage is stable, the charging time constant is R3C3, and C and U are fixed values, the average value I of the current passing through the milliammeter is directly proportional to the opening and closing frequency f of the contact, so the reading of the milliammeter can directly reflect the engine speed.