Talking Voltmeter

 

Ever needed to measure a voltage without taking your eye off something? If so, then this is the project for you! The Orton Talking Voltmeter takes a voltage measurement in the range 0-5 Volts and then reads it out to you to two decimal places (i.e. to 10mV resolution). The design illustrates the use of the 10 bit ADC in the PIC, as well as pulse width modulation (PWM) for analogue output. This project also illustrates how to squeeze every last word of program memory out of a PIC! Sound samples consume almost all of the on-chip flash memory, and even that was after some pretty brutal trimming of the samples and a drop in resolution to seven bits. Even after all that, the samples are about as brief as you can get without becoming unrecognisable.

Necessary Files

The circuit is so simple it seems almost a crime not to use all of those digital I/Os. The internal ADC of the PIC derives its reference from the supply lines, and to that end a variable voltage regulator is employed to permit full scale calibration. If 5% accuracy is acceptable, then a fixed voltage regulator could be used instead. When calibrating the unit, always set the pot for 5V VDD BEFORE you install the PIC, as the setting may be on the high side to start with. Final adjustment should be done with a known reference attached, which may be nothing more than a battery if a DVM is around to tell you what reference its providing.
The speaker drive shown in this circuit is simple but has a number of drawbacks: Firstly, it is inclined to interfere with nearby radios due to the rapidly switching currents. Secondly, the power consumption is quite high – about equivalent to Class A – and that is too high for battery power to be practical. Lastly, the high level of PWM signal will probably drive your dog nuts! A less minimal but perhaps more conventional approach uses a linear audio amplifier to drive the speaker. If that option is preferred then the output filter supplied with the circuit can be used to obtain a line level signal for the amplifier input.
Plain perforated board is ideal for this sort of project (see reverse side photo). Component leads can be easily formed into ‘PCB traces’ with tinned copper wire making up when these won’t reach. Vero pins (can you still get those?) provide anchor points for the leads.
If nothing else, this project has served to remind me how very unpleasant it is to hear your own voice coming back at you like something demented. My best authoritative voice too! Oh no. It just struck me. I sound like my school headmistress.