How do you build a simple timer circuit?

Answer 1

Timing is complicated in electronics, and there are only a few good solutions.

You should use a microprocessor called the LM555 timer (or just the 555 timer). While seemingly daunting at first, this small 8-pin device affords a lot of features and can perform one-shot timing (press a button and keep a light on for a certain time determined by the resistors and capacitors), astable timing (essentially generating an on and off signal), bistable timing (controlling two lights; one on and one stage, and one on at another stage), and with some clever 'hacking' it can be used to encode an analog signal into a PCM (pulse-code modulation) signal, often used for devices such as remote controls and toy aircraft.

A quick overview: There are the pins 4 and 8 which should be powered. When you use pins, you count from the bottom left, which is indicated by the small indented circle in the chip. Pin 1 should be connected to the negative end of the battery or power supply. The chip can run from 2 volts to up to 14 (some chips support 18) volts but for something like an LED, 9 volts should be good (a different voltage would mean you'd need a different resistor for the LED). Pin 5, optionally, should be connected through a small capacitor (100nF or near) to the negative end of the battery; the capacitor should not be electrolytic. This is optional as it just helps settle the value on one of the internal comparators and makes the timing more accurate.

Pin 7 should be connected through a 10k resistor to positive. Pin 6 should be connected to pin 7. At the end of this series, you should connect a capacitor, which determines the timing amount (as well as the resistor). The second capacitor should be about 100 to 220 uF, which will give you a range of 1.1 seconds to 2.42 seconds. You can of course adjust to any value suited. Pin 2 is the trigger. A 10k resistor should be wired though a push-to-make switch, connected to the negative end of the battery. Then, the positive end of the push-to-make should be wired into pin 2. Pin 3, the last pin, is the output. Wire a 330 ohm resistor and a green LED into this and you are set. Wire the end of the LED to ground (the negative end). Press the button, and it should work, timing for the value of the capacitor. The formula for the time is T=1.1(CR) where C is the timing capacitors' value (C being in farads, and it will be very small), and R being the resistor connected, in ohms. This will be the larger value. The 1.1 multiplication is just because of how the timer works.

The whole circuit can be build on a breadboard/prototyping board. Or, if you have the skills, you can build it on a printed circuit board.

It should be noted however, this timer is not 100% accurate. Because of component tolerances, you should allow for as much as 17.5% inaccuracy in both directions. If you need more accuracy, you need to get into the very complicated world of quartz crystals, real-time clocks and the like; and even they suffer from 'clock drift' (being incorrect by up to 30 seconds a month depending on temperature). In short, if you need accurate timing, you need an atomic clock!

There are many resources on the web for how to do this. Simply use Google or your favourite search engine to search for '555 timer'.