#122 Op-amp Timer
Test an op-amp timer switch.
Notes
This circuit uses an op-amp configured as a comparator to switch an output for a period of time governed by a variable RC time constant (𝛕).
The comparator switching point is half the supply voltage, set by the R1/R2 voltage divider. So with a 5V supply, the switching point is 2.5V.
At steady-state, C1 is fully charged to positive supply and this sets the inverting op-amp input well above the non-inverting threshold and therefore the op-amp output is off (low).
When S1 is pressed, C1 is fully discharged and the inverting op-amp input pulled low, so op-amp output switches on (high).
When S1 is released, C1 charges through R4+R5, and the op-amp output remains high until the C1 voltage reaches the switching threshold. R4 is a 50kΩ pot, which allows the switching time to be adjusted. R1 is included to set a minimum resistance and avoid a short if R4 is dialed to 0Ω.
The time it takes to switch can be determined from the RC complete response formula. Assuming a 5V supply:
v(t) = v(∞) + [v(0) - v(∞)] e^(-t/𝛕)
2.5 = 5 - 5 e^(-t/𝛕)
2.5/5 = e^(-t/𝛕)
ln(0.5) = -t/𝛕
t = - 𝛕 ln(0.5)
So the low and high timing limits are:
Note that the switching logic may be flipped to normally on with a timed off period by switching the inverting/non-inverting inputs.
The LM741 is not capable or rail-to-rail operation. In fact it is not a particularly good op-amp - but it is convenient. With a 5V supply, the particular unit used here only measures a 2.6V swing:
- output low: 1.6V
- output high: 4.2V
Here’s a sample trace of the circuit behaviour recorded using LEAP#090 PlotNValues (a simple Processing sketch)
- upper trace is the op-amp output
- lower trace is the non-inverting input (i.e. C1 voltage)
Construction
Note: the Arduino is included for measurement purposes only. The circuit does not require it. When a 5V Arduino like the Uno is attached, the circuit should also be powered at 5V. Without the Arduino, the supply could be anything within the operating limits of the op-amp, with perhaps R3 adjusted to limit current to the LED as required.
Credits and References
- LM741 Datasheet
- talkingelectronics timer circuit - a similar circuit that inspired this build (with a few modifications)