Posted on Oct 6, 2012

An electrical signal taken from the low tension side of the distributor is converted into a voltage proportional to engine rpm and this voltage is displayed on a meter calibrated accordingly. The 555 timer IC is used as a monostable which, in effect, converts the signal pulse from the breaker points to a single positive pulse the width of which is determined by the value of R4 + RV2 and C2. Resistors R2 and R3 seta voltage of about 4 volts at pin 2 of ICl. The IC is triggered if this voltage is reduced to less than approximately 2 volts (Ys of supply voltage), and this occurs due to the voltage swing when the breaker points open

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An adjustment potentiometer RV1 enables the input level to be set to avoid false triggering. Zener diode ZD1 and the 180 ohm resistor stabilize the unit against voltage variations.

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