Forrest M. Mims III: Voltage Probe with Tone and LED Outputs


By Forrest M. Mims III

A voltage probe can be very handy when troubleshooting circuits. The one described here indicates a positive voltage with both a tone and a glowing LED. It is powered by the circuit being tested, so it needs no onboard power supply. It can be built for a few dollars and can be easily modified.

Electronics DIY Project

How It Works

The circuit uses CMOS 4011 quad gate. Gates 1 and 2 form an audio oscillator with a frequency determined by C1 and R3. The signal from the oscillator drives a piezo speaker. Gate 3 serves as a buffer that drives the LED. In operation, IN serves as an input probe. Power is applied by connecting +V and ground to, respectively, the positive supply and ground of the circuit being tested. When the signal at IN is low or when IN is not connected to an external circuit, R1 pulls the input to ground (low) and keeps both the oscillator and the LED off. When the IN probe is high, the oscillator emits a tone and the LED glows.


Circuit for the voltage indicator
Figure 1. Circuit for the voltage indicator


Parts You Will Need

The following parts were used to assemble a breadboard version of the circuit:

Part Description Mfr. Part No.
IC1 – 4011 quad NAND gate CD4011
R1, R2 – 1M resistor CF1/4W105JRC
R3 – 100K resistor CF1/4W104JRC
R4 – 1K resistor CF1/4W102JRC
C1 – 0.01µF capacitor DC.01
LED – any common red or green LED LH2040 (red) or LG12340 (green)
Piezo speaker element TV4 or TV1

Miscellaneous: Perforated prototype board, 9 volt battery, battery connector clip, double-sided tape, wire jumpers, alligator clips or or mini-clips (select from the wide variety available at www.Jameco.com).

Note: While the components listed above were used for the prototype, substitutions can be made.

Prepare the Board and Install the Components

The circuit was assembled and tested on a solderless breadboard and tested. When the circuit was operating properly, the components were transferred to a 2.9 x 1.25 inch (7.5 x 2.9 cm) board cut from a perforated prototype board and soldered in place. The board should be cut from the end that has parallel rows of three and four connected holes. (Don't use the end of the board with parallel rows of two connected holes.)

If you want to make a permanent version of the circuit, you can follow your own parts layout.

Note: You might consider installing the circuit in a small enclosure. Or you can simply copy the layout shown in Fig. 2 to make a trial version of the circuit.

Layout for an assembled version of the dual output voltage probe
Figure 2. Layout for an assembled version of the dual output voltage probe


1. Begin assembly by orienting the board so that the uncut end (the side with three connected holes) is the bottom side.

2. Insert the IC into the top of the board with pin 1 (indicated by a dimple in Figs. 2 and 3) is in hole B53 (the rows and columns are marked on the back side of the board). Bend pins 1 and 8 slightly outward to hold the IC in place.

3. Insert a bare wire jumper into the holes adjacent to pins 5 and 6. Bend the ends on the back side of the board slightly outward to hold the jumper in place.

4. Repeat step 3 to install a jumper into the holes adjacent to pins 12 and 13.

4. Install an insulated jumper (purple wire in Fig. 2) over and across the IC between holes Y52 and Z58. Bend the ends of the jumper slightly outward on the back side of the board.

5. Install an insulated jumper (yellow wire in Fig. 2) between holes X51 and Z51. This jumper should be bent toward the closest edge of the board to leave hole Y51 open. Bend the ends of the jumper slightly outward on the back side of the board.

6. Continue assembly by installing C1, R1, R2 and R3 as shown in Fig. 2. As with the jumpers, bend the leads slightly outward on the back side of the board.

7. Refer to Fig. 2 and insert R4 across holes C56 and B57.

8. Refer to Fig. 2 and insert the leads of the LED into holes C58 (anode) and Y57 (cathode). The cathode of the LED is indicated by a flat spot in the base of the component.

9. Use a low-wattage soldering iron to solder all the wire leads to their respective foil patterns on the back side of the PC board. Caution: Be sure to work in a well ventilated room when using lead solder.

10. Put on some safety glasses and clip off the excess lead lengths. Inspect your work carefully to make sure there are no solder bridges between adjacent lands. Caution: Wear eye protection when using a wire cutter to trim leads and pins.

11. Bore a small hole around 1/8-inch (3 mm) in diameter at hole T57.

12. Use double-sided tape or an adhesive to mount the piezo speaker on the board as shown in Fig. 2. Thread its connection wires through the hole at T57.

13. Solder the red piezo speaker lead to the foil strip between holes Y51 to Y54 on the back side of the board.

14. Solder the black piezo speaker lead to the foil strip between holes V51 to V54 on the back side of the board.

15. Solder the red 9-volt battery clip lead to the foil strip between holes B56 and B58 on the back side of the board.

16. Solder the black 9-volt battery clip lead to the foil strip between holes V51 to V54 on the back side of the board.

17. Remove 1/2 inch (1 cm) insulation from one end of a jumper wire and insert it into hole E53. Bend the lead over to the foil strip between holes B51 and B54 on the back side of the board and solder in place.

18. Carefully check the circuit for any wiring errors or missing solder connections. Remove any solder bridges between foil traces using a solder sucker or braided solder remover.

Testing the Circuit

Connect a 9-volt battery to the battery clip. If the LED glows and/or if a tone is heard when the input probe is not touching anything, quickly remove the battery and carefully check your wiring. Look for solder shorts between the PC traces and component leads that might be touching one another.

When nothing happens when a 9-volt battery is connected, touch the input probe to the positive battery terminal. The LED should glow and a tone should be heard.

The completed circuit does not use a self-contained power supply. When the circuit is working properly, clip off the battery clip (adjacent to the clip), remove 1/4 inch (8 mm) insulation from the free end of each lead and solder alligator clips or mini connector clips to the wires. These form power supply connectors for the circuit.

To use the voltage probe, connect the power supply clips to a circuit being tested (red to positive and black to ground). Then touch the probe to various points in the circuit to determine if a positive voltage is or is not present.

Going Further

The logic probe can be installed in a suitable enclosure for permanent use. If you decide to keep the circuit for permanent use, ground the inputs of the unused gate of IC1 (pins 8 and 9). Leave the output at pin 10 open. You can easily alter the frequency of the output tone by changing the value of C1 or the resistance of R3.

About Forrest M. Mims III