User's Manual

UT5 - 9
UNDERSTANDING THE PULSE INPUTS:
In this instance, a "pulse" is a millisecond application of either a positive or negative voltage from a digital output
device, or just the simple punching of a momentary button switch. The choice of positive or negative pulse inputs is
provided for your convenience. If you want to start the timing with a push button or microswitch, simply connect the
switch to points C and D.
THE RESET LINE:
Once triggered, the monostable timer IC will continue its timing cycle even if it's triggered again. To RESET the timer
in the middle of an unwanted timing cycle, apply a negative pulse to the reset terminal [B]. When the reset function is
not needed, it should be connected to [F] to prevent false triggering.
A NOTE ON R3 AND R4:
In both monostable and astable modes, all timing is controlled by C1 and the combination of R4 (fixed) and R3
(variable or fixed). A simple "starter" potentiometer is supplied with your kit for R3. You can install any value or style of
trimmer or panel-mount pot that suits your application. Don't hesitate to exchange R4 to a value that puts you more "in
the ball park" than may be possible with the R3 pot that you are using. For example, if your application calls for the
timing resistance to be around 500K, you can either use a 500K or 1 meg control for R3, OR you can replace R4 with
a value such as 470K.
STANDARD CAPACITOR VALUES:
It's fine to calculate an exact capacitance in theory, but we have to work with fixed value capacitors which are actually
manufactured. Also, remember that capacitors are manufactured to tolerances that may seem quite broad for a
precision timing calculation. For both reasons, R3 is adjustable, and its exact value can be checked on a multimeter of
a reasonable quality. Also, resistors are available in very precise tolerances, if necessary.