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Teeny Timer: a low-tech timer for your car There are plenty of applications in a car where a simple timer is required. This one doesn’t use any ICs or even a PC board. It just uses a transistor, a capacitor, a relay and very little else. You could wire it up in almost no time at all and get delays of up to 30 seconds. By LEO SIMPSON This simple circuit came about because one of our readers wanted a timer for his water-cooled turbo intercooler. The timer was to be used to control an electric water pump and was to operate for a set time (seven seconds) each time after it was switch­ed off. No doubt you can come up with a dozen other uses. Now we could have come up with a fancier design using an IC such as a 555 timer. But that would have required a PC board and this reader 24  Silicon Chip just doesn’t feel at home with ICs and PC boards. He also wanted the timer to operate in the engine bay and that ruled out consumer versions of the 555 or other timer ICs since their maximum operating temperature is only 70°C. OK, we thought, “how do we come up with a simple timer, not using an IC and the absolute minimum of parts?” Oh, that was the other requirement: he didn’t want a lot of parts in it because he gets confused when soldering them up! He’s a pretty demanding customer, this one. Anyway, we put the feet up on the desk, thought of faraway places, anything but timers really and finally this idea popped into the vacuum: “use a transistor”. The result you can see in the circuit of Fig.1. How it works The heart of the Teeny Timer is a Darlington NPN transis­tor, Q1. When current is fed to its base via the 10kΩ resistor, the transistor is turned on and the relay is actuated to operate whatever you want. The diode across Top of page: the Teeny Timer uses one Darlington transistor, a 1000µF capacitor and not a lot else to provide delays of about 38 sec­onds. A bigger capacitor would give a longer delay. Note that the circuit is wired on lowtech tagboard. Fig.1: when switch S1 is closed, the +12V rail is applied to the 1000µF capacitor and the 10kΩ base resistor of the transistor, to turn the relay on. When switch S1 is opened, the 1000µF capaci­tor discharges via the 10kΩ base resistor to provide a fixed time delay. the relay is there to absorb the backEMF generated by the relay when it turns off. The circuit operates as follows. When switch S1 is closed, it feeds +12V to the 1000µF capacitor and the 10kΩ resistor. This turns on the transistor and operates the relay. When the switch opens, the 1000µF capacitor continues to feed bias current to the transistor and so the relay stays on until the capacitor is substantially discharged. There’s not much more to it than that. The +12V supply is fed via an in-line fuse. Varying the delay The time delay can be varied by varying the size of the capacitor. On our version, the 1000µF capacitor gave a delay of about 38 seconds. 470µF would give about 17 seconds; 220µF about seven seconds; 100µF would give about four seconds and anything smaller you wouldn’t bother about. Longer delays could be obtained by using bigger capacitors. For example, 2200µF should give about one minute PARTS LIST 1 12V automotive relay (see text) 1 SPST toggle switch (S1) 1 inline 3AG fuseholder 1 5A 3AG fuse 1 plastic utility case, 130 x 68 x 42mm 1 BD679 or BD682 NPN Darlington transistor (Q1) 1 1N4004 silicon diode (D1) 1 1000µF 16VW electrolytic capacitor (see text for value) 1 10kΩ 0.25W resistor 1 5-way tagstrip 1 grommet 1 screw and nut to suit relay Miscellaneous Spade lug connectors, hook-up wire, solder. 10 seconds while 4700µF should give about three minutes. The actual delays will depend on the capacitor tolerance, the gain of the transistor, the ambient temperature and the supply voltage and whether you’ve had too much to drink lately. Not precise enough for you? Hey, this is a low-tech, low-cost design. Building it No PC board! Whoa! Whaddya we do now? In a throwback to the dim distant past, we built the cir­cuit on a 5-way tagstrip. Fig.2 shows the wiring details. The relay is a standard automotive type with SPST contacts and rated at 20-30A. They can be purchased from any automotive parts outlet for about $11 or from Jaycar Electronics at $6.95. The relay is mounted with a single screw and nut, to the base of the plastic case. You can either solder the connections directly to the relay or fit the wires with spade lug connectors, as we did. Testing it This is the easy part. Connect the circuit up to a battery or 12V supply and operate the switch. The relay should operate immediately. Then turn the switch off. The relay should stay closed for your desired delay time and then give a click to show that it has opened. We mounted the whole lot in a plastic case and the leads all came in via a grommeted hole at one end. If you are going to mount the Teeny Timer in the engine bay of your car, keep it as far away from the hot spots as possible. Mount the case so that the wire entry hole is at the bottom, so that water splashes don’t become a problem. SC Fig.2: the wiring diagram shows all the details. Note the polari­ty of the diode and electrolytic capacitor and make sure you wire the transistor correctly otherwise it won’t go. April 1997  25