Digital AC Voltmeter

Posted on Feb 5, 2014

It started out as a simple unenclosed variac (variable autotransformer) and electrical outlet mounted on a block of wood. I eventually put it in the case of what used to be an old audio signal generator for safety and appearance. Among the other added features, I used a 0-250VAC analog meter that I had on hand to display the output voltage because otherwise

Digital AC Voltmeter
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it was annoying to whip out the multimeter to monitor the voltage coming out of the variac. Over the years I realized that the meter was inaccurate and simply useless, so a new readout was necessary. I wanted a digital AC voltmeter to measure the output range from 0 to 150VAC with reasonable accuracy. Sure I could buy some premade DVM packages or use a microcontroller with a built-in ADC, but I wanted to make one from scratch myself using readily available parts I had on hand. I aimed for reasonable accuracy so I chose to have a 2 and a half display for the voltage, meaning the meter reads from 000 to 199. Below is the schematic of the AC DVM: I used a voltage-controlled PWM circuit to convert an analog DC voltage to a pulse duty cycle. To do this, a 555 timer is used to generate a sawtooth wave at about 500Hz that is fed into a comparator to compare against the measured analog DC voltage. The output of the comparator has 500Hz period with a pulse width that is proportional to the input DC voltage. However, the sawtooth swing is only around 1. 65 to 3. 3V so a voltage divider between 5V and the measured DC voltage is used to offset and scale down the swing to this range. The 10K resistor on the sawtooth 555 timer is for adjusting the transistor bias to control the current going into the 0. 1uF capacitor. Adjust this until the sawtooth is as clean as possible and has the 1. 65-3. 3V range and the correct frequency. Be...

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