Électronique / Electronic
Here's a timer you can use for taking timed tests or simply for knowing when an amount of time has passed. You can preset this timer for as much time as you like up to about 15 minutes. When the time is up, it gives out a continuous buzzer sound until you turn off the power or press the key to reset the circuit.
After you build this project set the control to position 2 on the dial and slide the switch to position ON to turn on the power.
Hold a stop-watch and start it when you press the key. The timer makes a buzzing sound in about 30 seconds or more. Now set the control to each division on the dial from 2 to 8, and note how long it takes the timer to produce a sound.
Setting the timer's calibration, the time that passes at each setting of the dial-requires a lot of patience, but it is necessary for making sure your timer works accurately.
After you set the calibration, make a graph showing each control position and the time it takes for the buzzer to sound.
Now, your tester is ready for use. Look at the schematic. The control changes the reference voltage of the comparator (IC 1).
The timer setting is determined by the resistor R and the capacitor C. When the voltage applied to the positive (+) terminal of IC 1 exceeds the reference voltage, the alarm sounds. Since the dual operational amplifier has a high input impedance (input resistance), its current loss is very small, so you can use it to make a timer with a very long setting.
IC 2 works as an astable multivibrator that produces the buzzer sound.