Momentary Contact Switch

I need a dependable momentary contact switch for a low voltage circuit, but I want to spend very little money on it. I made a momentary contact switch that can be adapted for a variety of configurations. Shown is a very basic form for demonstration purposes. A key part of it is a non-conducting spring made from a piece of a common nylon cable tie.

Materials and tools are given in regular print and in italics. Those italicized concern the second part of this Instructable in which I built a momentary contact switch for a trigger mechanism on a non-contact voltage tester (NCVT).

Materials

1. Nylon cable tie
2. Wood block or other mounting material.
3. Brass screw
4. Wire
5. Hot glue (optional)
6. Plastic electrical tape
7. 1/4 inch white plastic Plexiglas
8. 12 gauge copper wire
9. Brass sheet
10. Solder
11. Small sheetmetal screws

Tools

1. Drill and drill bit (or thin saw)
2. Side cutter pliers
3. Knife
4. Fine saw (hacksaw)
5. Screwdriver
6. Soldering gun or iron
7. Pliers

Drill a hole into the mounting material (or wood) at a low angle. The diameter of the hole should be sized so the nylon cable tie fits into it fairly well, but not too loosely. A thin saw about the thickness of the cable tie could be used to make a mount for the cable tie, as well.

Cut the thin leader on the cable tie's end away and place the blunt end of the cable tie into the hole from the previous step.

Wrap bare copper wire around the end of the cable tie. Use plastic electrical tape to secure the wire to the cable tie. Put on a bead of hat glue if there is a need to insulate the conductor electrically. Drill a hole into the mounting material for the brass screw and insert the screw. Make a loop on the end of the other wire after stripping the insulation from it and tighten the brass screw to hold the wire. The nylon cable tie insulates and provides a spring that returns to its original position when released.The bare end of the wire wrapped around the end of the cable tie could be tinned with solder for more permanence. Be careful not to damage the nylon cable tie with too much heat.

I have used commercially available non-contact voltage testers (NCVT), but I have forgotten how much time had passed since changing the batteries. The batteries in them leaked and ruined the testers. So, I built two of the circuits at this Instructable. I power one of them based on a 4017 chip with two 2032 batteries. Those have never leaked for me. I powered the other with a 9 volt battery. Those batteries also have not leaked for me.

I found the battery drains a little and goes "dead" when not in use on the circuit based on a 555 timer after a couple of weeks. I added a pushbutton momentary contact switch to both circuits. See the photo for one I mounted inside a plastic pill bottle. But, I have not had good experiences with those pushbutton switches. They quickly become very rough in their action and electrical contact through the switch becomes unreliable. I wanted an alternative I can operate smoothly and reliably with a trigger mechanism, and decided to make my own. What follows adapts the basic momentary contact switch in the preceding steps of this Instructable.

Someone gave me some white plastic Plexiglas 1/4 inch in thickness that I am using to make a pistol grip NCVT with a trigger switch. The photo shows cutting a piece for the trigger from a piece that will become part of the pistol grip. The table saw is my home made adaptation of a good 7 1/4 inch circular saw. I have used this table saw adaptation for many years and find it very accurate and reliable.

I have attached the pistol grip handle to the flat piece pf Plexiglas on which I will mount the circuit assembly. The piece for the trigger switch has been laid in place for fitting. The top front corner of the trigger touches the circuit mount to function as a stop for the trigger on its return. The trigger is cocked so their is a gap at the top rear to provide space for it to rotate.

In the second photo I have drilled a pivot hole through the trigger and the pistol grip behind it.

In the third photo the pivot screw for the trigger has been added with a washer. Leave it just loose enough that the trigger moves smoothly. A piece of plastic has been cut to be part of the pistol grip in the same layer with the trigger. The photo was taken in the process of marking and cutting to fit around the trigger while leaving a gap for internal electrical parts, etc.

The fourth photo shows the part of the pistol grip that fits around the trigger with a nylon cable tie laid over it to decide where it should be mounted to act efficiently as a return spring for the trigger.

The first photo shows the trigger piece next to a piece of 12 gauge copper wire. The wire has been bent to fit into two holes in the back edge of the trigger piece. (The two holes are difficult to see because they are white on white with very little shadow.) The second photo shows the 12 gauge wire set into the holes in the trigger piece. The holes were angled to help hold the wire in place. I also soldered a connector wire to the 12 gauge wire. The connector wire passes through a hole large enough to allow free movement when the trigger is pulled. The third photo shows the internal parts of the trigger activated momentary contact switch. Toward the lower part of the photo is the spring action provided by the cable tie piece. Its upper end will extend as the trigger is pulled, so leave space for it to move. The other conductor is a piece of brass sheet that has a concave bend on each end formed into it. Thin saw cuts have been made in the plastic to receive and hold the brass strip in place. The other lead is soldered to the brass strip.

Another thickness of plastic is screwed to the handle to keep all of these pieces in place, but is easily removable in case they do not stay in place. I had to thin the backside of the plastic covering the switch internals a little to be certain the trigger does not catch or hang up from friction.

I used a piece of scrap plastic to cover the internal parts of the trigger switch, mostly to keep them from falling out. If anything goes wrong, I can remove the cover and make modifications. With an NCVT there may be concerns that a high voltage under test could somehow enter the NCVT circuitry and cause electrical shock to the user. Covering the parts with another layer of plastic provides extra protection. I did thin the overlay piece of plastic a little to ensure the trigger can operate without friction from the overlay piece.

This is a physically larger NCVT than most pocket commercial units, but I use it like I woud a digital meter. That is I am not carrying it during the day in a shirt pocket, but reach for it like I do for larger tools. For me, its size does not matter. A smooth, dependable trigger switch action matters far more than its size.

If I were to make this again, I would make the trigger piece wider. The spring action from the cable tie is very adequate, but a second piece of cable tie layered with the first to make the trigger pull heavier would not be unwelcome and would make accidental engagement of the switch less likely. A sufficient gap between the back of the trigger piece and the plastic layer in the handle behind it would need to be provided.

The graphic is the schematic I am using for my NCVT based on a 555 chip. It includes some modifications I made based on experience. As mentioned I increased the battery size to 9 volts because such batteries are so available and are easily kept away from the circuitry to avoid damage in case of leaking. If you forget to check on the battery after several months, the battery will simply go dead. Even if it leaks corrosion will not reach the rest of the circuitry. The extra voltage also provides more than enough power for good sound on the buzzer. A 555 chip can work on a range of supply voltage that exceeds 9 volts.

Somewhere I read it is good practice to tie pin 4 on a 555 timer to pin 8 for stability. And, the original circuit linked above has a 220 Ohm resistor connected to pin 3. Then the buzzer and the LED connect to the opposite end of the resistor. The purpose of the resistor is to reduce the current going to the LED to protect it. I do not want to reduce the performance of the buzzer, so I connected the buzzer and the current control resistor directly to pin 3. The buzzer bypasses the resistor. And, I added a momentary contact switch to isolate the battery from the circuit when not in use.

In use, I find I need to wave the antenna slowly near the conductor being tested for live current. This tester may display a flash from the LED when the switch in engaged, even with a conductor that has no live current. It may even remain "on" briefly. But, the LED is steady and does not flash unless live AC current is present in the conductor under test. A lot about sensitivity depends on the antenna you use (connected to pin 2). You can experiement with the length and shape of the antenna for your best results. Most of these antennae are not longer than about six inches, but mine currently is almost double that and it works fine.

In case you are wondering, the 555 timer operates in astable mode in this circuit.