Nuts and Volts - February 2018

quite close to the calculated value.

The second section of the 556 is wired as a monostable multivibrator and should have a quite short output pulse. The formula for the pulse width of the monostable section is PW = 1.1*R3*C3 = 110 µs. When the trigger signal goes low, the 555 output immediately switches high and the capacitor starts to charge. However, since the trigger is low for longer than the calculated pulse width, the capacitor does not charge completely. It only charges to about 0.7V due to Q1 being on.

Picture 5 shows the results with and without Q1. A is the triggering pulse from the astable; the actual triggering edge is off the screen to the left. B is the voltage across C3 when the transistor is in place.

You can see that the capacitor voltage is clamped at about 0.7V until the triggering signal goes high, at which time the capacitor is allowed to complete its charging. The capacitor charges only to the threshold voltage, which then enables the internal discharge circuit.

C shows the capacitor voltage with Q1 removed. It charges to the power supply voltage since the discharge circuit is disabled, as long as the trigger is low. The slope of the discharge is due to the finite amount of current the discharge circuit can pull from the capacitor. Using the formula cv=it, the current calculates to about 14 ma: .01 µF 5V/3.55 µs. The only difference between having the transistor in the circuit vs. out of the circuit is that with the transistor, the output pulse is about 97 µs wider that without it. This is somewhat shorter than the 110 µs calculated previously due to the capacitor starting from .7V rather than 0.

Since the output pulse width on pin 9 is basically the same as the trigger pulse from pin 5 — except that it’s inverted — the same effect can be achieved with a 555 using the same timing components as Section 1 of the 556 by connecting the piezo element between the output and Vcc. If you use the circuit as a warning device, you can use the Reset input to enable and disable the oscillator.

To use the PIC replacement, you can work the circuit shown in Schematic 7. The schematic shows Range 3 (1 ms to 1 sec) and mode 5 which are probably the best for this application. You can adjust R6 to give you the desired tone/chirp duration, and R5 to set the repetition rate/period. If you use mode 4, then R5 would adjust the time between chirps.