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Figf. Z Schematic of the timer circuit: 68 73 Magazine * November, 1984

altered, other than to cut wires, (2) There was to be no power consumption when the power-control switch was off. (3) The existing pilot light should indicate when power was applied to the defroster grid (4) It should be possible to reset or turn off the defroster timer at any time, (5) The circuit should be able to cope with power-supply variations and noise.

Circuit Description

Fig. 2 shows the circuit that evolved from the design goals, A 555 timer chip was chosen as the control element R2 and C2 are the timing components The values shown produce an on-time of approximately four minutes. This has proven to be adequate for most situations. Increasing the value of R2 will increase the on-time, and vice versa The relationship between R2 and the on-time is roughly linear: doubling R2 doubles the on-time.

I have shown power-supply connections as going to + 14 V. These connections are really to the positive side of the car's battery, and the voltage may not be exactly 14. The circuit will function over a wide range of supply voltages, however, so this is not a problem

The 555 is triggered by a low voltage (less than one third the supply) at pin 2. C3

and R4 ensure a low voltage at pin 2 for a fraction of a second after power is applied. R5 guarantees a rapid discharge of C3 when power is removed, allowing the circuit to be reset and restarted quickly. R1 and CI are a simple supply filter. The output of the 555 (pin 3) will be at zero volts when off (or when power is removed) and at about one volt less than the supply voltage when on.

There are at least three ways to configure the defroster drive circuit (R6, Q1, B1, KU Figs, 3(a), 3(b), and 4 show the choices. With the circuit of Fig. 3(a), the exact timer output voltage does not make much difference; the key is to drive Q1 into saturation. When Q1 is saturated, its collector-to-emitter voltage is very small, so the voltage applied to the relay and indicator light is nearly the supply voltage In addition, power dissipation in the transistor will be minimized.

If the circuit of Fig 3(b) is used, Q1 will not be driven into saturation but will act like a larg^signal emitter follower Available drive for the relay will be about 2 volts less than the supply voltage due to the baseemitter drop across Q1, the drop in R6, and the limited maximum voltage at the 555 output Power dissipation in Q1 will be increased be-

cause of increased collector-to-emitter vol tage.

I recommend using the circuit of Fig, 3(a) when possible, but when the connections to the relay coil are difficult to access, it may be easier to use the configuration of Fig. 3(b).

Use Without a Relay

Due to the high current in most grid-type defrosters (10 Amperes or more), it is not advisable to drive the defroster directly with this circuit. If it is desired to eliminate the relay, the circuit shown in Fig. 4 may be employed. The added transistor, Q2, is to boost the current applied to the base of Q1 to ensure that it is saturated (minimum power dissipation}, Q1 still may dissipate several Watts, so a heat sink should be used.


Layout and construction style are not critical. I chose to use a small piece of punched phenolic board for the timer and point-to-point wiring.

R2 and C2 are the only components with somewhat critical values. Most other components can be ot whatever values exist in your junk box. Every one of the timers I have constructed used different component values! Just make sure Q1 will saturate and that the potential on pin 2 of the 555 will rise above 5 volts after C3 charges.


Finding a convenient, location for the timer may be the most difficult part of the project especially if a factory-installed defroster and switch are to be modified. There is usually very little extra space behind the dashboard, and Murphy's Law says that if there is extra space it will be located so as to be least useful.

If a relay is not used, an acceptable solution might be to mount the timer right at the defroster grid's terminals or just on the other side of whatever hole the wires go through. Since the circuit can be made reasonably small, it might not be objectionable if left in the open.

After-market add-on defrosters should make for an easier job. There is probably enough room near the switch to install the circuit.

Many defroster switches have an indicator light incorporated into the same kl


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