How you can Construct a Contact Sensor Circuit


On this venture, you’ll construct a contact sensor circuit. It’s a cool and easy circuit that means that you can management an LED with the contact of a finger. And also you solely want three parts, how cool proper?

You’ll be able to construct this circuit should you’re a complete newbie.

The Contact Sensor Circuit Diagram

You solely want a resistor, a transistor, and an LED to construct this venture. Under you’ll discover the schematics for the contact sensor circuit:

Touch sensor circuit schematic

Parts Wanted

  • 9V Battery
  • 9V Battery Clip
  • Breadboard
  • Jumper Wires
  • 470Ω Resistor
  • Pink LED
  • NPN Transistor (BC546/547/548 or related)

These are all commonplace parts you will discover at most electronics outlets.

How It Works

Your physique has electrical resistance. You’ll be able to test by measuring the resistance between your fingers with a multimeter.

So while you contact the wires of the touchpad, your physique behaves as a resistor. This implies a small quantity of present can stream from the plus of the battery, via the transistor’s base, and activate the transistor.

When the transistor is on, present can stream from collector to emitter of the transistor and the LED will activate. Study extra about this within the article on how transistors work.

How you can Construct the Contact Sensor Circuit

You’ll construct this circuit on a breadboard. However when you’ve examined that it really works correctly, be at liberty to go a step additional and solder it.

Step 1: Join The Transistor

Begin by connecting the transistor slightly bit down on the board.

In the event you’re utilizing a BC546/BC547/BC548 transistor, you possibly can join it as proven within the determine. The collector goes into row 10, the bottom in row 11, and the emitter in row 12.

Notice: When you’ve got a special transistor mannequin (ex the 2N3904), you must test the datasheet to seek out out which leg is which, and regulate your connections accordingly.

Breadboard with transistor

Step 2: Join The Resistor

Subsequent, join the resistor. Join it from row 1 to row 9.

Discover that it’s not linked to the transistor because the transistor is linked in row 10.

(Within the following pictures, a 220 Ω resistor is proven. Nevertheless it might be a good suggestion to make use of a bigger worth like a 470 Ω resistor to ensure your LED can deal with the present.)

Breadboard with transistor and resistor

Step 3: Place The LED And Touchpad

Now, you’re going to attach the LED and the touch-sensing half. For the touch-sensing half, you’ll want two uncovered wires. You may get this from the LED. The LED has for much longer legs than wanted.

Reduce off the legs of the LED in order that solely round 1 cm / 0.4 in is left on the LED. Bend the 2 legs that you simply simply minimize off, as proven under, in order that they slot in between 4 columns on a row.

LED with legs cut off to make touchpad

Place one on row 4 between column A and D. And one on row 6 between column A and D. This leaves room for a jumper wire to be linked later.

Insert the LED with its constructive facet on row 9 column F, and the opposite facet on row 10 column F. Now the LED is linked between the resistor and the collector of the transistor, identical to within the schematics of this circuit.

Breadboard with transistor, resistor, LED, and touchpad.

Step 4: Join The Touchpad

Now you must join the 2 contact wires to the right factors within the circuit, utilizing jumper wires.

Join one jumper wire from row 1 column F to row 4 column E. And one jumper wire from row 6 column E to row 11 column I.

Breadboard with the touch sensor circuit connected, without power connections.

Step 5: Contact It!

Subsequent, join a jumper wire from row 1 column J to the constructive provide column on the fitting. Then join a jumper wire from row 12 column I to the unfavorable provide column on the fitting.

Join your battery connector and plug within the battery.

Breadboard with the complete touch sensor circuit connected to battery

If the whole lot is linked correctly, nothing ought to occur proper now.

However then, attempt to contact the contact wires in order that your finger touches each wires on the similar time, and the sunshine will activate.

In the event you can’t see the sunshine, attempt making the tip of your finger moist earlier than touching once more. When the pores and skin of your finger is moist, the resistance decreases in order that extra present can stream via your finger and to the bottom of the transistor.

What If There Is No Gentle?

On this circuit there are three frequent errors:

  • the orientation of the LED is improper
  • the orientation of the transistor is improper
  • you might be utilizing a PNP transistor as a substitute of an NPN transistor
  • your finger is simply too dry

The NPN and the PNP transistors look precisely the identical, so the one technique to distinguish them is to have a look at what’s written on the transistor. If it says BC557, it’s a PNP transistor, which is *not* appropriate. If it says BC546 (or BC547 or BC548) it’s NPN and it’s appropriate.

In the event you tried all the above and it’s nonetheless not working, test the resistance worth of your resistor. It ought to be 470 Ω. If it’s way more, it restricts the present to the LED an excessive amount of. If it’s a lot much less, it might need damaged the LED.

When you’ve got any questions on this experiment, use the remark subject under.

The Outcome

Within the video under you possibly can see the method of constructing this, and the ultimate end result exhibiting the working circuit:





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