I am designing a circuit which will output a square waveform for a specific duration of time (t) but I need to be able to read the square waveform using a processor so would like to turn the changing square waveform into a single high pulse for time (t). The frequency of the waveform could be too high for the processor to read directly so I am looking for a method to convert it to a single high pulse (with duration t). I first thought about using a small capacitor which would be charged up during the rise of the square waveform and then be discharged through the low part of the waveform but I am unsure if this would work and what value the capacitor would need to be. Ideally I need way which does not use ICs (simple discrete components only would be best).
Any ideas?
Dan
How to convert a changing input (square wave) into a single high signal
frequencypulsesquarewaveform
Related Solutions
This is the simplest way without the 555 timer. You have to play with the RC values and the gate's input should be schmitt triggered.
simulate this circuit – Schematic created using CircuitLab
simulate this circuit – Schematic created using CircuitLab
Figure 1. Flying start 555 oscillator.
I've only used a 555 twice in the last few decades but this may do the trick for you.
- C3 is connected between V+ and the timing capacitor. On power up C3 and C1 form a capacitive potential divider. If the voltage rises smartly the junction of C3 and C1 will rise to \$V_+ \frac {C_3}{C_3+C_1} \$ thus priming the capacitor for you. So, to aim for 1/2 supply set C3 = C1. Bear in mind that R1 and R2 will be discharging C3 during power up so you may have some calculations or experiments to do.
- Once the oscillator starts up the timing will depend in C1 in parallel with C3 so use value \$C = {C_1+C_3} \$ in your timing calculations. The fact that C3 is connected to V+ rather than GND won't matter.
Best Answer
You need a retriggerable one-shot, like the 74HC423 The first rising edge will make the output high, and each subsequent rising edge will keep it high. The output will not go low until there is a time period between rising edges that exceed the output pulse width.
In the world of logic chips, one-shots aren't the greatest of ideas, and are considered to be cheats in many cases, but here it seems to make OK sense.