Unintended circuit delays can be a real nuisance. However, controlled delays may prove quite useful in some situations, as they allow actions to start at a predefined time. A microcontroller comes in handy in specific applications, but a simpler option is to use an arrangement of resistors, capacitors, and transistors to elicit the proper time response.
Whichever route you choose depends on a wide variety of factors specific to your application and needs. A timing circuit is an option to keep in mind for future design choices. Check out the diagram below for an example of one of these circuits.
Resistors left to right: 470R, 20K, 1K; 100μF capacitor; 2N2222 NPN transistor
Here’s a quick guide to how a capacitor timing circuit operates:
- The main differentiator with our circuit is that with the timer button open, there’s no difference in potential between the leftmost (Rl) and middle (Rm) resistor. Electricity can flow into the base of the transistor, allowing current to also flow through the transistor’s collector and emitter, energizing the LED in series with the rightmost (Rr) resistor.
- When the switch closes, one side of the Rl connects directly to ground, creating a sudden difference between the legs of the capacitor.
- Current then flows through the Rl to ground, while a lesser amount of current flows through Rm due to its larger resistance.
- Instead of supplying power to the transistor’s base, the capacitor intercepts this current and absorbs the charge because of the potential difference in its two poles.
- Once sufficiently charged, current again flows to the base of the transistor and out the emitter to ground, allowing current to flow from the collector to the emitter of the transistor and supplying power to the LED.
The time it takes a capacitor to charge fully is a “time constant” called “tau.”
Tau = resistance of the circuit (measured in ohms) times the capacitance (measured in farads)
This value signifies the amount of time it takes the capacitor to get to 63 percent of its charge value. The transient response time, or the time the capacitor takes to charge fully, is equal to 5 times this value.
Since we’re using a 100μF capacitor and there is a resistance of 20K in the circuit, the time constant is .0001F x 20,000R = 2 seconds. Multiply that value by 5 and you have a capacitor charge time of 10 seconds.
However, things here aren’t quite so simple. Upon experimentation, you’ll find that the time it takes for the LED to light up is closer to 1.5 seconds than the 10 you might initially expect. This is because the transistor’s base only requires a relatively small amount of current in order to reach saturation, and the capacitor’s charge absorption drops off precipitously after approaching a single time constant. The diverted charge therefore powers the lights in less than 2 seconds.
Double capacitor circuit variation
Of course, this is far from the only capacitor timing circuit available. You can also use a different size capacitor to experiment with the delay time of this circuit. In a pinch, you can even wire several in parallel.
