Nuts and Volts - July/August 2018

semiconductor process used for the part: MOS versus bipolar. Using an MOS part (input current about 10 pa) allows you to pick a photoresistor with higher on and off resistances. If you are using a bipolar part, you will need a photoresistor with lower resistance values and you may need to make R3 smaller.

You can use the circuit of Schematic 2 for this application as well, with either astable mode ( 4 or 5). The same photoresistor circuit can also be utilized to drive the gate/trigger (RA3) input.

Mims Circuit 25

Schematic 9A is an infrared security alarm. U1 is configured as an astable multivibrator with a frequency of about 6 Hz, period = 166 ms. The LED will be turned on when the output is low; about 3. 2 ms. U2 is configured as a monostable multivibrator (one-shot) with an on-time of almost 52 ms.

With the insert in place, Q1 will be off which causes Q2 to be held off and the trigger input of U2 is held high. With the trigger high, the output will be low keeping the buzzer turned off. When the insert is removed, U2 will be triggered as well as Q2 turning on synchronously with the output of U1 causing the one-shot (U2) output to go high.

Capacitor C2 will charge until it reaches the threshold voltage at which time the output will turn off and the discharge circuit will turn on. The buzzer in this circuit must be of the internally driven type.

Since I was unsure of the function of Q2, I tested the operation of this circuit by driving the U2 trigger circuit directly from a PIC 555 in mode 4, range 2, and off-time of about 3 ms.

When the trigger goes low (simulating the insert being removed and LED1 turning on), U2 is triggered, Q2 turns on, the 555 discharge circuit turns off, and the buzzer turns on for the duration of the C2 charge cycle.

With the component values shown, the removal of Q2 has no impact on the circuit. The output low time of U1 is short enough ( 3. 3 ms) that Q2 does not affect the charging of C2. If the U1 output low time is increased to 6 ms or greater, then Q2 begins to have an effect on the charge time of C2; see Picture 1.

The area circled in red shows the plateau caused by Q2 which is clamping the capacitor to about 0.5V.

Since the plateau does not start until about 5 ms after the trigger, it will not occur with a trigger pulse width less than 5 ms.

Once the trigger is removed, Q2 will turn off allowing C2 to finish charging to the threshold voltage and turn off the output.

Schematic 9B shows another possible implementation. This one is similar to FM- 24 in that the Reset input is used as a gate for the astable multivibrator.

I also changed the operation such that the frequency of the 555 is about 500 Hz so that the buzzer needs to be of the externally driven type. In this case, the buzzer will sound continuously when the insert is removed.