In this instructable we will be making a touch lamp from recycled components.
You will need:
- matrix board,
- one bright LED, or few if you are using a power transistor,
- minimum four general purpose NPN BJT (Bipolar Junction Transistors) - 2N2222,
- resistors (100 ohm, 1 kohm, 10 kohm, 100 kohm, 1 Megohm),
- wire stripper (optional),
- AA battery holder for two batteries,
- hole drill or electric drill,
- screw driver (optional),
- soldering iron and solder (optional - you can twist the wires),
- encasement for light (you can use an old lunch box or used food container),
- metal wire (0.9 mm),
- blue tag or masking tape,
- multi-meter (optional - you can use a LED or light bulb),
- heat sink (this is really something that you might not need unless you are using high power LEDs).
Step 1: Build the Circuit
In the circuit above the LED is modelled with three general purpose diodes because PSpice software is old and does not have LED components. The voltage across the LED must not exceed 2 V.
"The larger end inside the led is -ve and the shorter one is +ve, that is how we find out the polarity of a LED." https://www.instructables.com/id/IDENTIFY-THE-ANOD... Current must flow from positive to negative terminals or from anode to cathode to turn on the LED because LED conducts in only one direction.
You should check the transistor pin out. This can be done by searching for transistor specifications on the internet. If you connect the transistor the wrong way you might burn it.
All transistors have three terminals. A BJT transistor has base, collector and emitter terminals. The arrow pointing forward is the base emitter terminal in the circuit diagram shown because current is flowing from base to emitter.
In NPN BJT transistor a small current, Ib is entering the base terminal and a higher current, Ic (usually a 100 times more, depending on transistor current gain) is entering the collector terminal. Emitter terminal can be connected to ground as shown in the diagram with (Q1a, Q1b and Q1c transistors). The collector terminal input can be modelled as a current source with value, Ic = Ib * Transistor Current Gain. Thus the voltage across the collector is equal to: Vce = Vs - Vled - Ic*Rc1. In the diagram there are three transistors. That means Ib is split into three equal magnitudes (if all three transistor characteristics are all the same - which never happens in practise) and then higher current magnitudes from Rc1 resistor and the LED are split into three equal magnitudes (if all three transistor characteristics are all the same - which never happens in practise) to flow into the three collectors.
The base emitter voltage should be no more than about 0.6 V and the collector emitter terminals should not be connected directly to battery to prevent transistor failure. You might do this accidentally if you connect the transistor the wrong way during testing (because you can supply 0.5 V to base emitter terminals without burning the transistor).
You should test the circuit during construction if you are not an expert. Be patient.
Instead of three transistors driving the LED you can use a power transistor so that you can connect more LEDs in parallel. Each LED will need a 100 ohm resistor. Do not use one resistor for two or more LEDs. This might cause one LED to be more darker than other. I tried this experiment my self. Each LED has its own turn on voltage dur to manufacturing tolerances. Some LEDs turn on at 1.9 V while others a 2.1 V. This if the LEDs are placed in parallel and the voltage is only 2 V, the second LED might be a bit dark.
The heat sink for the power transistor is optional. It might cost a lot of money because you might want to use a heat transfer paste. Usually the transistor should be saturated, leading to very low power dissipation is Ps = Vce * Ic = 10 mA * 0.2 V = 2 mW.
The circuit is showing a cascaded transistor amplification. The LED might turn on with only two transistors. Infact if you put the two terminals in water the LED might turn on slightly without any transistors when connected in a series circuit with high voltage battery. However, we are building a touch lamp, not water indicator. Thus we need high current amplification. Also, your fingers could be dry and not conduct electricity properly on certain days.
Step 2: Testing the Circuit
The circuit diagrams shown are optional solutions for higher voltage power supplies for more LEDs and more brightness or changing the circuit if it is not working.
Most likely the circuit will work and will not need any modifications.
When you test the circuit there could be two problems:
Problem 1: The LED us always on.
This would happen if you transistors have high current gain. Try reducing the circuit to just two transistors as mentioned in the previous step. You connect the base of Q2 transistor directly to one of terminals. Do not do with if the LED is not on all the time. The circuit might work with two transistors for you but not another person.
This would happen if you transistors are very sensitive. Try reducing the supply voltage to amplifiers because transistor current gain falls with power supply voltage. You see a variable resistor in the circuit diagrams shown (RsensitivityA variable resistor). A good option is to include the variable resistor in the circuit to control the LED brightness even if the circuit is working.
Another problem could be oscillations in the power supply or EMI (electromagnetic interference) entering the base of Q3. Oscillations in the power supply are caused by high LED current causing a voltage drop at the positive battery terminal. This voltage drop occurs because all power supplies have internal resistances and when the load resistance falls a voltage across the positive terminal falls according to voltage divider law:
Vl = Vs*Rl/(Rs+Rl)
Where: Rl = Load Resistance (ohms), Rs = Internal Resistance (ohms), Vs = Battery Voltage (V), Vl = Load Voltage (V).
A drop in power supply voltage caused by amplifier output LED current will cause a drop in current entering the base of each transistor which is amplifier input. This you have a feedback loop and thus power supply oscillations. This is why higher Rb values are used for initial cascaded amplifier stages rather than final amplifier stages to draw less current from power supply (I = V/R) when possible and be right value for the RC filter resistor values with R needing to be ten times less than load resistance for each RC filter output.
The basic RC filters at the top are used to filter power supply oscillations. The cut off frequency is equal to: f = 1/(2*pi*RC). The remaining RC filters entering placed between the transistor emitters and bases are used to filter EMI entering the Q3 transistor. It is highly unlikely that you will need those filters because I did not use them.
Problem 2: The LED is not turning on.
This would happen if you transistors have low current gain. Increase the power supply voltage because gain increases with supply voltage. You can see other circuit diagrams showing high power supply voltage that can be also used to drive more LEDs to allow more brightness.
Add additional amplifiers as shown in the other circuits.
It is highly unlikely that you need those two solutions. Low transistor gain might mean that old components that you are using could be close to failure.
Step 3: Drill Hole Terminals
You use the electric drill, hole drill and scissors.
If is hard to choose the right drill diameter for the bolts which you will screw in the next step. Thus scissors are always a good option to widen the holes.
Warning: Do not apply high pressure and do it slowly. You might cut you hands.
Step 4: Attach Touch Terminals
This step will require pliers or screw driver, depending on the type of bolts that you are using.
Step 5: Drill Circuit Attachment Holes
You do not need scissors for this step. A tiny hole would do the job unless you are using a thick metal wire. However, even then you can just use a thicker drill because the holes does not have to fit the wire exactly. However, a thicker wire might not fit the matrix board holes for next step and you might need to drill the matrix board which is not really a problem.
Step 6: Attach Circuit to Case
Use 0.9 mm mental wire to attach circuit to case as shown in the photo.
Step 7: Attach Battery Holder to Case
Use blue tag or masking tape to attach the battery holder to case as shown in the photo.
You are now done.
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Trash to Treasure