Touch Lamp

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In this instructable you will be making a touch lamp. The lamp should turn on when you touch the two terminals with fingers or two hands.

You will need:

- encasement (for example: cardboard box, old food container, old light encasement),

- matrix board,

- solder and soldering iron (unless you want to twist the wires together if you have space for a large matrix board),

- NPN and PNP transistors (2N2222 and 2N2907a) - do not choose low power or low current transistors (you might want to purchase a few in case you accidentally burn the transistors or break the legs),

- bright LED,

- hole puncher tool or drill,

- wire stripper,

- scissors,

- resistors (check the circuit),

- wires,

- bolts, nuts and washers (two of each),

- pliers for bolts and nuts,

- 2 AA batteries,

- 2 AA battery holder with switch,

- metal wire (1 mm thickness),

- metal wire (0.5 mm thickness),

- 9 volt battery with battery harness (optional),

- additional 2 AA batteries (optional),

- additional 2 AA battery holder without switch,

- crocodile clips (optional),

- multi-meter (optional),

- screw driver for bolts, battery holder or both (optional),

- nail for making holes (optional),

- 500 ohm or 1,000 ohm variable resistor for trouble shooting (optional).

Step 1: Build the Circuit

The checking of the circuit is in the next step. However, you should check the circuit during construction.

You can short circuit the two terminals. The circuit should still work after that.

Warning: Make sure you turn off power when soldering because soldering iron is grounded.

This circuit could have been done with an operational amplifier or a comparator. However, those devices need higher voltage, at least 6 V, or 30 V across power terminals (according to specifications).

In this circuit you are using NPN and PNP BJT (Bipolar Junction Transistors). Those transistors have three terminals. Base, Collector and Emitter. The emitter is the one with the arrow in the circuit. The base is the input to the transistor and the collector is opposite the emitter. BJT Transistors are current amplifiers. The current entering the base is amplified by the transistor and thus usually about 100 times less than collector and emitter currents. Collector and emitter currents are almost equal.

Each transistor will have its own pin out configuration. The transistors in the photo might not have the same pin configuration as the once you purchased. You should google the transistor datasheet to see the pin out diagram.

The voltage across the base emitter terminals should be about 0.7 volts. This is how you can tell that you connected the transistor in a correct way. Do not connect transistor terminals to power supply without putting a resistor in series with terminals. You will burn the transistor.

The bright LED only conducts in one direction, from anode to cathode terminals if you are assuming that current is travelling from positive to negative battery terminals. In reality current travels from negative to positive terminals. This is called electron current. However, electronic engineers assume the conventional current as travelling in the opposite direction to avoid a headache.

The anode pin is longer: https://learn.sparkfun.com/tutorials/polarity/diode-and-led-polarity

In this circuit diagram shown the bright LED is modelled with three general purpose diodes because the PSpice software simulator does not have LED component models. I only use this simulator because of keyboard short cuts. I spend ten times less time drawing the circuit in this software than I would if I used a website simulator or any other software.

The voltage across the LED must not exceed about 2 V. If there is a resistor connected in series with the LED the voltage across the LED would not usually exceed about 2 V, unless the resistor value is very low and power supply is very high. However, the current across the LED must not be above about 10 mA. If the power supply is 3 V and the voltage across the LED is 2 V then the voltage across the Rc1 resistor will be about 1 V and the LED current is 1 V / 100 ohms = 10 mA.

You can add another LED with another 100 ohm resistor in parallel. However, do not connect more than three LEDs in parallel or high current LEDs. You might burn the transistors. This is why I used 3 transistors to reduce the power dissipated for each transistor. A light bulb of high current LED will require a power NPN transistor with heat sink.

The transistor current gain is influenced by manufacturing tolerances, collector emitter voltage, collector current, ambient temperature and ageing. The minimum transistor current gain is usually about 20. During on operation the collector of Q2 transistor is about 3 V. Thus it is not possible to calculate the total collector current of Q1 transistor: (3 V - 0.7 V) / 1000 ohms / 3 * 20 * 3 = 46 mA. Thus this the three transistors are able to power four LEDs in worse case scenario.

The value of Rb2 is not very important. You can try a number of values. The minimum must be 2,200 ohms because you might burn the Q2 transistor base. However, large resistor values will increase device lifetime. The 1,000 ohm Rb1 value is for 3 transistor bases. If you know the resistor colour codes you can see that in the circuit photo I did not use the same resistors as in the simulation file because I simply did not have them in stock.

Rc3, 1 Megaohm resistor is to make sure the collector of the Q3 transistor is connected when the terminals are not connected, or off state of the circuit. Removal of Rc3 might cause the bright LED to turn on event when you not touching the touch terminals with Q3 transistor picking up electromagnetic interference. The current entering Rc3 resistor must be very small to avoid amplification loss.

Step 2: Test the Circuit

You can short circuit the two terminals. The circuit should still work after that.

It is not likely that circuit will need modifications. If the circuit is not working the most likely reason could be due to faulty components or because you did not connect the components properly.

However, here are some of modifications that you can do if your circuit is not working:

Previously I did mention that transistor gains are influenced by currents, voltages, ambient temperature or ageing.

Two possibilities might happen after you test the circuit:

Problem 1: The bright LED is not turning on - the transistor current gains are very small or your fingers are dry. Do not wet your fingers. The circuit should work with dry fingers. An LED can turn on with a simple series circuit connection if your hands are wet.

Solution 1: Transistor current gain increases with power supply voltage. Try increasing the power supply voltage to 4.5 V, 6 V, 7.5 V or 9 V. Vary the 100 kohm resistor to increase the supplied voltage. Look at the circuit diagrams shown.

Solution 2: You can connect another cascaded transistor to increase the current gain as shown in the circuit diagrams. However, connecting another transistor will cause the circuit to amplify electromagnetic interference (EMI), keeping LED on all the time. Also, there are costs involved of using extra circuit components. Thus increasing the power supply voltage or variable resistor solution is better choice. However, solution 2 might allow the circuit to operate at lower power supply voltages.

Problem 2: The bright LED is on all the time. This might happen because the transistor current gain is too high or there is too much electromagnetic interference (EMI) from a nearby industry, power station or radio station. The general purpose transistors cannot conduct high frequencies because of stray capacitance across the terminals which filters those frequencies. However, there could be high power low frequency EMI.

Solution 1: Transistor current gain increases with power supply voltage. Try reducing the power supply voltage. Vary the 100 kohm resistor to reduce the supplied voltage.

Solution 2: You need to connect an RC filter to filter the electromagnetic interference (EMI) in the cascaded chain. The cutoff Frequency = 1/(2*pi*10,000 ohms*(1*10^-6) Farads) = 15.92 Hz. The turn on time will equal to 5 time constants: Ton = 5*10,000 ohms*(1*10^-6) Farads = 50 ms.

If you are using 3 V from bench top power supply or switching power supply there could be oscillations in the power supply voltage. Those oscillations can occur due do to 50 Hz or 60 Hz AC mains frequency oscillations or switching power supply oscillator. Those oscillations are usually very small in magnitude. However, they will be amplified by entering, Q4, Q3, Q2 and Q1 transistor bases and collectors. Thus in the circuit diagrams shown you see optional power supply filters. It is not likely that you will need those power supply filters. I did not need them in my circuit. The cutoff Frequency = 1/(2*pi*10 ohms*(1000*10^-6) Farads) = 15.92 Hz. Larger resistor values are chosen to reduce capacitor values further down the cascaded chain. Thus you need to make sure that Rb1 < 10*Rb2 < 10*Rb3 < 10*Rb4. Otherwise the last transistor amplifier, Q4 will have a very small potential voltage.

Step 3: Drill Touch Terminal Holes

You might try using power drill or hole puncher tool.

Do not use nail with hammer if the plastic is hard. You might cut yourself. If you are using an old food container you can apply gentle pressure with a nail but only if the plastic is soft.

Next step is widening the hole with scissors if you drill or hole puncher is too narrow. Make sure you do it slowly and gently. Do not apply high pressure.

Step 4: Attach Touch Terminals

You connect the wires to bolts with washers and nuts as shown in the photo. You might want to use special connectors for wires to bolts available on internet. However, they might be expensive and not necessary.

The problem is that the circuit might move when you open the box or shaking the device when you walk. The movement causes the wires to move and we all know that when you bend metal many times, the metal breaks. So all wires might secured.

Put the wire in parallel with the bolts or connectors (that you might have used instead of washers) and wind thin metal wire (0.5 mm thickness) across the bolts or terminal connectors.

Once you attach the wires to terminals you might find that the wires are picking up electromagnetic interference (EMI) and the LED is on all the time. This is not likely if you are using only three transistors. If you are using shielded wire, the EMI might come from the terminals alone. However, this is very unlikely at all.

Step 5: Drill Circuit Attachment Holes

You might try using power drill or hole puncher tool.

Do not use nail with hammer if the plastic is hard. You might cut yourself. If you are using an old food container you can apply gentle pressure with a nail but only if the plastic is soft.

Next step is widening the hole with scissors if you drill or hole puncher is too narrow. Make sure you do it slowly and gently. Do not apply high pressure.

Step 6: Attach Circuit to Case

You use 1 mm metal wire to secure the circuit to case as shown in the photo.

You can see in the photo that the power supply and touch terminal wires are secured with 0.5 mm thickness wire to matrix board to avoid breaking.

Step 7: Attach Battery Holder

Use blue tack or stick tape to attach battery holder.

You are now done.

Watch this video of the device in operation:

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