For checking the control function of the timer IC, set potmeter VR1 again in the maximum resistance position. Also set preset VR3 in the minimum resistance position. Trigger the monoshot using switch S1. You’ll observe its output going high for a time period that is much less than that determined from the series combination of R8 and VR2, and capacitor C4. In fact, for any fixed setting of this series combination, the output pulse width can be observed to vary for different values of potmeter VR3 resistance—by triggering the monoshot several times, once for each setting of VR3.
Now set the DPDT switch in position 2-2. LED1 and LED2 glow alternatively with the timing determined by the resistances in the charge and discharge paths. This means the timer IC is okay and wired in astable mode.
The functions of reset and control pins can be checked in astable configuration too in the same way as discussed above for the monoshot configuration.
The article was first published in July 2005 and has recently been updated.