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Cheapest way to build this circuit?

mabo

New Member
How much will it cost to build this circuit in the absolute cheapest way possible? Basically, what I need it to do is this:
A button that you can touch "through the glass", and that button will light up an LED light.

Can I make this circuit for less than One US dollar?

How can I make the signal travel thru the glass and directly to the LED light? Do I need an Arduino for this? I am hoping to find the easiest and cheapest way for this.

The link below shows a circuit that does the following:

You push the button "though the glass", then it sends a signal to an Arduino, then it sends the signal to a doorbell to make it ring. It has some extra things that I do not really want, but it is shown here for demonstration.

Through-Glass Doorbell
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The simplest version of something like that uses two opamps to create a capacitive touch switch, then it needs a transistor to switch power to the bell or bell relay.

No microcontroller or anything complex.

It would need a metal foil patch on the inside of the window or door (which could not be double-glazed).

I'll try and come up with a circuit tonight if I get chance, unless someone else beats me to it.


Another option would be an optical switch working by bouncing light off the finger. That's more complex but also longer sensing range.
 
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dr pepper

Well-Known Member
Most Helpful Member
You can get very cheap microcontrollers these days, I cant suggest a super cheap one however, you can get them with capacitive touch inputs on some of the pins, might be worth looking into.
 

DrG

Active Member
You did not give a lot of details, but you may want to look into these capacitive switches.


TTP Datasheet

I bought them a while ago. They are dirt cheap and they do work (more or less). You can interface them to a uc or use them stand-alone.

Specifically: They have three pins; Vcc, GND, Out.

You can set them to jog or toggle mode (there is a solder bridge on the board)

With Vcc=5V, GND=Ground and out connected to a 150 ohm resitor to one side of an LED and the other side of the LED to ground (I don't know/remember how much current the board OUT signal can sink).

In jog mode: LED is lit on power up, touch pad and LED is off - remove finger and LED is back on.
In toggle mode: LED is lit on power up, touch pad and LED is off - remove finger and LED stays off, touch again and LED is lit.

You don't actually have to touch the pads and they would work through a non-metallic barrier, like glass. How thick, I don't know - 1-4 mm, probably yes, 20 mm, no.

You can apparently, use it with one of those 5 V relay boards that operate on a low (see advert), again I don't know how much current that example relay requires. Maybe that would be a good way to implement a doorbell.

I used some with an Arduino clone in a silly project.


Again, they are cheap and can be noisy if you are not careful....maybe even if you are.
 

Externet

Active Member
The cheapest... I would try a LDR glued to the back side of the glass and a transistor to drive the LED. Sort of a night light lamp, which is available to the public for 99cents and has more (housing) parts. -Battery not included-
The finger shade/blocking light will do the trick. No complications.
Borrowed from the web:
120507
Unsure about 330 Ohm, but if it is an optocoupler instead of the LED, multiple possibilities open.
 

mabo

New Member
H
You did not give a lot of details, but you may want to look into these capacitive switches.


TTP Datasheet

I bought them a while ago. They are dirt cheap and they do work (more or less). You can interface them to a uc or use them stand-alone.

Specifically: They have three pins; Vcc, GND, Out.

You can set them to jog or toggle mode (there is a solder bridge on the board)

With Vcc=5V, GND=Ground and out connected to a 150 ohm resitor to one side of an LED and the other side of the LED to ground (I don't know/remember how much current the board OUT signal can sink).

In jog mode: LED is lit on power up, touch pad and LED is off - remove finger and LED is back on.
In toggle mode: LED is lit on power up, touch pad and LED is off - remove finger and LED stays off, touch again and LED is lit.

You don't actually have to touch the pads and they would work through a non-metallic barrier, like glass. How thick, I don't know - 1-4 mm, probably yes, 20 mm, no.

You can apparently, use it with one of those 5 V relay boards that operate on a low (see advert), again I don't know how much current that example relay requires. Maybe that would be a good way to implement a doorbell.

I used some with an Arduino clone in a silly project.


Again, they are cheap and can be noisy if you are not careful....maybe even if you are.
Great find thank you!

Can you let me know if i need anything else for this project? What I need the project to do is this. I want to touch the glass panel, so that it activates the switch (TTP223B), and that causes the LED light to turn on, as long as the switch is being touched. As soon as you take your finger off the switch, the LED light should turn off again.
These are all the parts I think are needed for this project. Do I need anything else? Do I need to have an Arduino or other things not mentioned here on the list below?

List of items:

A small glass panel - $0.40
TTP223B - $0.58
LED light - $0.05
small battery- $0.12
small wires for connection; less than $0.10

So if this is correct, then all that the TOTAL COST for this project is about $1.20

Is this correct? Can you tell me if I am forgetting something?
 

DrG

Active Member
I don't know why you are trying to do this or what all the details are...frankly, it sounds like some kind of an assignment and I feel like I (we) am (are) doing it for you. Not crazy about that and it would help if you communicated something along the lines of understanding what is going on.

Nonetheless, here is what I just did.
120518

I put a 5V power supply onto a common breadboard. The TTP323B sensor board is wired as Vcc to 5V, GND to Ground, and, Out goes to a 330 Ohm 1/4 w resistor. The other side of the resistor goes to the anode of a green led and the cathode of the led goes to ground.

I put a glass pane (from a small picture frame - probably 1/16 in or maybe 1/8 in thickness) over the sensor as shown in the picture.

I turned on the power. When I touch the glass by the sensor or hold my finger near the glass by the sensor, the LED lights and stays lit as long as my finger is in that small area (see pic below).

120519

When I move my finger away from the area, the led turns off (see below).
120520

Note that in this example, the OUT signal of the capacitive sensor board is driving the LED through the 330 ohm resistor. It can source enough current (probably dropping voltage to supply it) to light the led.

While this satisfies the assignment, it is not the way I would do it in "real life" for a door bell or buzzer. You have omitted the resistor and you need to know why it is there - so, could you at least do that much please? You have also stated a "small battery". If the voltage of that battery is too high, it will not work and will probably damage the sensor. If the voltage is too low, it will not work. If the voltage is ~ 5V and it can't supply enough current (something like about 20-30mA, I would guess), it will not work.
 
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mabo

New Member
Thank you. Here is something I found :
TTP223B Digital Touch Sensor Capacitive Touch Switch Module

TTP223 Touch Sensor Tutorial Without Microcontroller

In these 2 videos, they connect everything without a microcontroller. What is the point of having a microcontroller if you decide to add it to the circuit on here?
 

DrG

Active Member
What is the point of having a microcontroller if you decide to add it to the circuit on here?
This works much better if you, as the thread starter, bother to answer questions that are posed to you...like this one..

You have omitted the resistor and you need to know why it is there - so, could you at least do that much please?
As for your latest question...There would be no reason to add a microcontroller to the circuit, UNLESS, you planned on doing something else besides turning the LED on/off.

You repeatedly asked if you needed an Arduino

Do I need to have an Arduino or other things not mentioned here on the list below?
Do I need an Arduino for this?
But, nobody said that you needed an Arduino. In fact, several posts, including mine, showed several approaches for doing it without a microcontroller.

Yes, post #3 did mention microcontrollers that have capacitive inputs - I don't think you are ready to understand that, but it was not an reasonable unreasonable suggestion.

I get it, you are new to that stuff.
 
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gophert

Well-Known Member
Most Helpful Member
I get it, you are new to that stuff.
Just because someone is new to a topic, they must have some experience asking questions, listening to answers and replying to follow-up questions. I think you are too kind.
 

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