Projects: 555 Based Digital Voltmeter


This project is a remake of my greatest college project, the digital voltmeter. This time I made a major modification.

Instead of using a DAC for producing the ramp I used the 555 timer to produce the analog linear ramp voltage for higher resolution. Later I realized that I could greatly reduce component count by using the 555 chip itself as a single slope converter for measuring input voltages.


Single Slope Analog to Digital Converter

To understand how the voltmeter works let me introduce the concept behind a single slope analog to digital converter.

single slope converter

The circuit below shows a simplified single slope converter circuit, analog signal to be measured is compared with a linear ramp of definite slope (peak and period). The output of the comparator is therefore a pulse signal whose duty / pulse duration is proportional to the magnitude of the input signal.

To quantify the pulse duration, the output of the comparator is fed to an AND gate. This output serves as a pulse gating circuit for pulses coming from an astable multivibrator. A counter is used to count the number of pulses from multivibrator when the gating pulse is high.

From this we can conclude that…

Vin proportional # of pulses

Vin = scaling factor * (# of pulses)

The frequency of the multivibrator and the slope of the ramp generator are carefully selected to have a scaling factor in the range of x1, x10, x100, x1k, and so on for easier approximation of input voltage without further processing (computation).

A voltage divider or amplifier with controlled gain may be used at the input to limit the maximum input to be equal to the ramp’s peak voltage.

I selected the ramp slope to be 0 – 5V in 500mS. Since I have a four digit display, an overflow of the display will occur on the 10000th pulse received, this is the maximum # of pulses that should occur while the ramp voltage is climbing from 0V to 5V.

Computing for the Astable Multivibrator frequency…

Frequency = max # of pulses / ramp generator period

Frequency = 10000 / 500mS

Frequency = 20kHz

Gain can be calculated as

Gain = ramp peak voltage / max Vin voltage


Ranges Gain Resolution
000.0 – 1000.0 mV 5 0.1mV
0.000 – 10.000 V 0.5 1mV
00.00 – 100.00 V 0.05 10mV
000.0 – 1000.0 V 0.005 100 mV


Dolly Single Slope Analog to Digital Converter:

I have to ask forgiveness from the entire hobby community for I know LM555 is the most popular IC when it comes to producing a reliable timing. The word “timer” used to be a precise description of this chip but everything changed when I began the remake of my project.

At first I intend to use this chip as a simple ramp generator for my Single Slope ADC but then I realized I could make my circuit much simpler if I reduce the number of comparators.

Using a chip for a different purpose is nothing new to me, and in fact I am known for this habit and this always happen when I get bored. Hehehe.

Dolly Single Slope ADC (Markup)

The “Dolly Single Slope ADC” is centered on the chip’s internal comparator originally intended for voltage control of the output frequency (the one whose inputs are the threshold (pin6) and the control voltage (pin5) ). An external buffer was added to prevent the loading effect of the internal divider network and present a high impedance to the input side.

A constant current source (LM317 but can be implemented using a simple transistor) supplies current on the capacitor to produce the ramp voltage which is compared against the input via 555’s pin6. The transistor Q5 resets the ramp when its amplitude exceeded that of the input voltage.

Another comparator was added because when the 555 discharges it will begin ramping up again at 1/3 of the supply voltage( 9V /3 = 3V), adding an external comparator that triggers at lower voltage (around 1V and can be less) extends the measurable range of the multimeter.

A larger view of the simulation result


The Display:

The display and counter module is based on 74C925 IC (of course anyone can hack a counter or a pedometer to cut the cost further). The rest of the components are trivial.

Counter Module


~ by glutnix_neo on February 21, 2011.

5 Responses to “Projects: 555 Based Digital Voltmeter”

  1. my 2nd entry to the contest…

    so love it! 😀 😀 😀

  2. corrected entries for the 2nd external comparator’s purpose…

  3. found a similar project while doing google. The only difference is the single slope comparator is outside the 555. 555 is used only as ramp source(like what I first conceived).

    still it’s cool.

    I will adopt the transistor AND gate on my version2, use a diode on pin2 trigger, will remove Q5, and then use a transistor CC source. wow this would be a highly stripped down version. 😀

  4. Contest has ended, too bad I didn’t win any major price. I realized I lack videos and pictures of the prototype.

    Though I didn’t won any major prices I’m very excited to receive a door price from Gabotronics…( Actually this is one of the items I wished I could have 🙂 )

    XMEGA Xprotolab

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