if you were not aware of it, the rush hour can affect your health. Stuck in a traffic jam or caught in the dense traffic in a big city, car drivers are often forced to switch off the fan to prevent being choked by the exhaust fumes produced by the vehicle in front of them. Switching the fan on and off every minute or so to keep the fumes out is a nuisance, and calls for an automatic switch controlled by the engine speed. Such switches exist, but unfortunately for most of you, they are only found in top-of-the-range cars equipped with an airconditioning sys-tem. The circuit shown here will not set you back too much, vet puts you at a par with certain BMW 7xx drivers—well, at least as far as the fan control is concerned. The control automatically switches off the fan when the engine runs at a relatively low speed. After a short delay, the fan comes on again automatically when you are clear to drive after passing a congested spot or start moving again in a traffic jam.
The circuit consists essentially of (1) an engine speed monitor circuit set to a switching level of 1.800 rev/min: (2) an integrator to prevent fast switching around the engine speed: and (3) a time constant
that intvod-Lices a delay before the fan is switched on. The engine speed monitor serves to sense
whether you are moving or not. This is achieved by detecting when the car engine idles, i.e., runs at a relatively low speed. The monitor consists of two monostable multivibrators. ICia and ICib. The first is supplied with pulses from the contact breaker. Resistors R1-R2 and diodes D1-D2 serve to reduce the pulse level to the maxi-mum supply voltage of the circuit. This protection is necessary because in some
cars the contact breaker pulses can have a peak value of up to 200 V. Monostable ICia supplies pulses of a fixed length as long as the period of the input signal is greater than the time constant defined by network
Pi-R3-C1. If the input pulses are shorter. the Q output of ICia remains at 1. The time constant, ti, depends on the number of cylinders. N. in the engine: = 120 / ( engine speed x N ) For example, for a four-cylinder engine, and a switch-off speed of 1 800 rev/min. t1= 120 / ( 1 800 x 4 ) = 16.67 ms.
