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Project Notes

#076 MonostablePulseExtender

Use an Arduino to test the behaviour of a 74LS122 monostable with external timing configuration and plot the results.


The 74LS122 monostable multivibrator in this circuit has pulse width programmed by selection of external resistance and capacitance values R2 and C2. Pulse width is given as:

tW = K * R2 * C2
=> where K is nominally 0.45. If C2 is in μF and R2 is in kΩ then tW is in seconds

For 33μF and 100kΩ then pulse width is 1.485s. This is intentionally large so that a visible pulse produced.

Once triggered, the basic pulse width may be extended by retriggering the gated low-level-active (A) or high-level-active (B) inputs, or be reduced by use of the overriding clear.

In this circuit an Arduino sends high-level-active (B) input pulses to trigger the monostable. LEDs are separately connected to output Q and Q complement, so they will toggle as the pulse goes high and low respectively.

The Arduino also reads output Q with an analog input pin and sends the data to PlotNValues (a simple Processing sketch). Here’s a trace of the behaviour. The lower trace is the trigger state, and the upper trace is the gate output.

processing trace


The Breadboard

The Schematic

The Build

Credits and References

About LEAP#76 CMOS/TTLArduino
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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

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Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.