This article is about an LED (Light Emitting Diode) voltage level indicator bar graph display.

You can see the my circuit consists of nine inputs. One input is the analogue input. The rest are digital inputs that I connected to digital decade counter. The device presented in this article is a one dimensional display of voltage versus time. The brightness indicates the magnitude of the voltage. Also, the LEDs will be OFF if the voltage is below zero. However, the brightness control potentiometer can offset the negative inputs.

The 1 Megohm potentiometer is used to control the brightness of my display.

You can see the circuit working in the videos shown.

Supplies

Components: BJT (Bipolar Junction Transistor) PNP general purpose transistor - 10, Power transistor - 2 (you might need spare), LEDs - 10, 1 kohm resistor - 10, 100 ohm resistor - 1, matrix board (10 cm by 10 cm).

Tools: wire stripper, pliers, drill, drill bit.

Optional components: heat sink, heat transfer paste, wooden panel or encasement, solder.

Optional tools: soldering iron.

Step 1: Design the Circuit

I drawn the circuit via Easy EDA software. You might think that I do not need the eight Q2 transistors and simply connect the LEDs directly to digital output. However, then I risk burning the LEDs if the voltage across the digital output exceeds 3 V. The eight Q2 transistors ensure that the voltage across each LED does not exceed 2 V. My circuit is more versatile can be connected to most digital inputs without burning the LEDs.

Step 2: Make the Circuit

Usually I do not use soldering iron to make circuits. I simply twist the wires underneath the matrix board. However, I had to use soldering iron for this project because there are many components on a very small matrix board and the density of components per square centimetre is high.

Step 3: Make Additional System Blocks

I purchased a bar graph display kit on Amazon for $9 including postage. This kit consists of a 555 timer square wave oscillator connected to a CMOS decade counter. The video shows a decade counter with 10 uF frequency controlling capacitor (connected to 555 timer). I later replaced the 10 uF capacitor with three 470 nF in series resulting in equivalent capacitance of 470 nF / 3 = 156.666666667 nF. The capacitor included in the Amazon kit was 1 uF.

I made a transistor inverter but I could have purchased an inverter TTL or CMOS microchip. However, I did not want to buy wire wrap sockets, wire wrap wire and wire wrap tool.

The output of decade counter in connected to input of inverter and the output of inverter is connected to input of the LED display circuit.