Holiday Special 19

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Hello folks,


As my this year's Holiday Special Project I thought it would be interesting to visualize the warmth feelings spread during the Christmas days. Therefore, feel welcome to meet my project "Touch of Love"


No matter if you want to conjure a smile on the lips of your partner or close-ones, with this gadget, you will have success. And the best part, you only need a few components and minimal soldering skills. Throughout this blog post, I will describe the circuit. First, from a general concept and then getting more and more detailed with each step. The passages next to the schematic contain more information regarding the design procedure and are good to know if you want to rebuild the circuit or modify it slightly. If this is not your intention, just skip them. Alright, enough talking, let's start...


As always, at the beginning of a project, there is some kind of vision of the end product. The below animation visualize this.

Fig 1. Touch of Love Animation.

Fig 1. Touch of Love Animation.

Credits to tenor and StefaniaDAngeli for base animation.



General Description

So the basic idea is to have a circuit that is going to react to any kind of body contact with the one whom you want to bring to smile or whom you want to show the strong bond between yourselves. From this point, we can deduce three states of the circuit:

  • Off - since this is a battery-powered circuit, a switch is used to turn the gadget on and off.
  • State 1 - After turning on the circuit we enter State 1
  • State 2 - Is triggered by body contact and transits automatically back to State 1 once the two persons do not touch anymore.

Having this general functional description, the next step is to give State 1 and 2 a more detailed description and to create a circuit that fulfills the correct state transitions.
I decided to use eight LEDs as a visual actuator which are turned on slightly in State 1 and are boosted with more current in State 2, resulting in a brighter glow.



Circuit Schematics

The circuit schematic is shown below:

Fig. 2 Toch of Love Schematic in Off State.







Let's investigate the circuit in a bit more detail.
The on/off switch S triggers the transition between the off State and State 1. Once the circuit is turned on, the default State 1 is entered and all LEDs are glowing slightly. The voltage supply (battery) is small in volume so an overall compact design can be realized. It is a coin battery with UBAT=3V.









For the description of State 1, let's focus on the below schematic with the active components marked in black.

Fig. 3 Touch of Love Schematic in State 1.   

As mentioned above all LEDs are glowing slightly. The glow is adjusted mainly by the resistance RLED. Choosing an appropriate value is affected by the color and amount of LEDs.

Detailed Design Description:
To calculate the size of RLED, we need to determine two values the current IRLED and voltage drop across the LED URLED.
IRLED depends on the type, color and amount of the LEDs. In my case, I used N=8 red LEDs with ILED=5mA at full brightness and IGlow=1mA. This means N LEDs in parallel require IRLED=N*IGlow=8mA for a slight glow.
Moreover, the LED forward voltage is ULED=2V. With this and the Kirchhoffs loop equation, we can calculate the remaining voltage drop over the resistance with URLED=UBAT-ULED = 1V. With Ohms law, we can compute a resistance of RLED=URLED/IRLED=1V/8mA = 125 Ohm.
Above explanation in short form:  RLED=(UBAT-ULED)/(N*IGlow)

Sidenote 1: The parallel connection is suitable for low voltage supplies and, therefore a compact gadget design because the number of batteries required is held low. Modifying the number of LEDs or the color requires only a change in the small resistance rather than a change in the bulky power supply.




Now that we investigated the circuitry responsible for the default glow lets have a look at the brightness boosting circuit.


Fig. 4 Touch of Love Schematic in State 2.

The idea behind the applied circuit is to increase IRLED by creating a second path for the current to flow. In the simplest version, you could do this by using a push-button in parallel to the resistance. Once the button is pushed the resistance, RLED is changed and the LEDs glow brighter. However, in this gadget, the switch should be replaced by body contact. As shown in the schematic, the body contact can be modeled with a very high resistance approximately a few mega Ohm. Such a high resistance conducts only a tiny amount of current which, applied directly to the LEDs, would not be enough to create a visible change. Therefore we need to amplify the current. This task is performed by the bipolar transistors T1-T3. Each stage is increasing the tiny current flow at the base of T1. Until a second LED current path is created at the collector-emitter of T3. The resistance RGlow is used to adjust the brightness boost.

Sidenote 1: Using bipolar transistors with a high amplification factor or Darlington transistors could cut down the number of components even more.

Sidenote 2: Using MOSFET instead of bipolar would be feasible as well, but because of the floating high impedance input at the first stage, the circuit becomes very sensitive to nearby electric fields. In other words, getting close to the gadget may already trigger the circuit, which in this application is not intended.







Enough theory for now, the assembly was done with the above components on a PCB prototype breadboard. First, I attached the LEDs and transistors in the desired shape and used the “legs” for connection. Afterwards, I soldered the two SMD transistors to the bottom side and added the switch. An old computer motherboard gave me, with a bit desoldering effort, a coin cell battery holder which I also attached to the bottom side of the PCB. Finally, two screw extensions are used to form a stable standing at the same time J1 and J2 for the touch response.
In the end, I used black paint to give a nice finish and insulate the bare wires on the back.

Fig. 5 Touch of Love - SideFig. 6 Touch of Love - FrontFig. 7 Touch of Love - Back




The result is shown in the video below. As already described at the top the circuit has a large smile factor. As the first “victim” I choose my girlfriend. First I switched it on and got the standard “Oh…” but when I told her to hold one side (while holding the other side myself) and put my hand into mine her eyes turned big “Woooow how is this working?!?!”



Have a happy new Year