Christmas only comes once a year, and when it does it brings a lot of joy.
A Christmas Series Lights Tester Circuit is an easy electronic setup made to check and fix problems in strings of Christmas lights that are connected in a series.
These light strings are often used for decorating during the holidays, but they can stop working if just one bulb burns out.
When that happens, it breaks the circuit and the whole string goes dark.
Below are the circuit diagrams with its working and constructions for Christmas Series Lights Tester Circuit.
Circuit Working:
The electronic solution is actually pretty easy, as shown in the circuit diagram above.
It allows you to quickly and easily find the issue without having to unplug all the bulbs or deal with other annoying tasks.
Plus, this device will be super handy throughout the year for testing fuses, lamps, power cables and more.
Any wire, like a string of Christmas lights plugged into the wall creates an electric field around it.
This happens even if no electricity is flowing through the wire, and it will still be there if there is a break in the string.
This circuit is made to find that break.
In the fig 1, you can see a set of lights connected to the power source with a break at point X because a bulb is either blown or not installed correctly.
Starting from the live terminal the electric field will be present at points A, B, C and so on until it reaches point X.
At point X there wont be any field which will help you identify the problem area.
After that you just need to check the bulb clean the connections, or push the bulb back into its socket.
If there are multiple faulty bulbs or connectors, the first one in the line will be the one that shows up.
Once you fix that one you can find the others easily.
However, there is a small issue with this setup.
Most sets of lights, if not all are wired like the one shown in the diagram and the bulbs are not spaced evenly along the entire length of the wire.
The long neutral wire is wrapped around the other wire making it impossible to test one without the other being there.
As long as the chain is plugged into the power source as shown and the lights are closer to the live end everything should work fine because the Live wire creates the electric field.
The neutral wire is almost at the same level as the ground so its electric field is very weak.
This means the tester will mainly pick up the field from the Live wire and if that field is missing it can be easily noticed.
If the neutral wire does break which is unlikely the problem will likely be with the lights or their sockets.
If there is a break at point X all the wires to the left will show an electric field while those to the right wont, since they will be neutral.
However, if the chain is connected the other way with the L terminal going to N and N to L there will be an electric field all along the chain, no matter if there is a break.
This is because the wire connected to L runs the entire length of the chain.
On Christmas lights both wires are usually green, so it is hard to tell which is which without untangling them.
If the lights do not work but an electric field is detected all along the cable, you should switch the connections by either flipping the plug for a two pin connector or rewiring the mains plug.
Detecting electric fields:
Parts List:
| Component | Specification | Quantity |
|---|---|---|
| Resistors | 1k | 1 |
| 100Ω | 1 | |
| Semiconductors | Transistor BC547 | 2 |
| Transistor BC557 | 1 | |
| LED | Any 5mm, 20mA | 1 |
An electric field can be detected by simply putting a conducting probe like a piece of wire or a metal plate into the field.
When the probe is in the field, it will take on the same electrical potential as the field at that spot.
If the voltage is alternating the field will also change back and forth.
The challenge is to create a useful display from this.
If you connect a regular voltmeter to the probe and the ground, it usually wont show a reading because it will overload the meter.
To get a good reading we need a high input impedance buffer or amplifier to drive an indicator.
This circuit is made up of a very powerful amplifier with high input impedance using just three transistors, two resistors and an LED as shown in figure 2.
The first two transistors are set up as a Darlington pair, which means their combined current gain is the product of their individual gains.
With the right components, this gain should be about 10,000.
The input impedance will be around 10 megohms.
The current from this setup goes straight to the base of a PNP transistor which then powers the LED.
If you want to hear a sound when the field is detected, you can also connect a piezo sounder.
This circuit can work with any voltage down to about 3V, so you can use batteries like a 9V PP3 or two 1.5V AAA cells.
However, at lower voltages the sound from the piezo wont be very loud.
The circuit uses so little power that you do not need an on/off switch and the batteries should last for almost their entire shelf life.
Formulas:
Below are the formulas for Simple Christmas Series Lights Tester Circuit:
Voltage Divider Formula at TP2
The resistor R1 1k and the base emitter junction of Q2 BC547 form a voltage divider.
The voltage at TP2 depends on the input at TP1:
VTP2 = VTP1 − VBE
where,
- VTP1 is the input voltage from the lamp terminal.
- VBE is the base emitter voltage of Q2 which is around 0.7V for silicon transistors.
LED Current Calculation
The current through the LED is determined by the resistor R2 100Ω and the voltage drop across it:
ILED = VTP3−VLED / R2
where,
- VTP3 is the voltage at TP3.
- VLED is the forward voltage of the LED which is around 2V for a LED1
Transistor Saturation Check
For the transistors to saturate Q1, Q2 and Q3:
IB = IC / β
where,
- IB is the base current.
- IC the collector current e.g. current through R2 or the LED
- β is the current gain of the transistor which is between 100–300 for BC547 and BC557
How to Build:
To build a Simple Christmas Series Lights Tester Circuit following are the steps to follow:
Gather all the components as mentioned in the above fig 2 circuit diagram.
Connect the base of transistor Q1 to TP1 tester, connect the collector of transistor Q1 to collector of transistor Q2, and connect the emitter of transistor Q1 to base of transistor Q2.
Connect the base of transistor Q2 to emitter of transistor Q1, connect the collector of transistor Q2 and Q1 to base of transistor Q3 as they are in Darlington mode, connect the emitter of transistor Q2 to GND through resistor R1.
Connect collector of transistor Q3 to GND through LED1 and resistor R2, connect base of transistor Q3 to the collectors of transistor Q1 and Q2, connect emitter of transistor Q3 to positive supply of +9V.
Connect a tester TP2 between emitter of transistor Q2 and resistor R2 and TP3 tester from the base of transistor Q3.
Safety Precautions
- Always ensure the tester is safe to use with the type of light string AC or DC.
- Disconnect the light string from mains power before testing to avoid electric shock.
Conclusion:
A Christmas Series Lights Tester Circuit is a useful gadget that helps you find problems in strings of lights that are connected in a series, like when a bulb is burnt out or there is a broken wire.
It makes fixing these issues easier and quicker, which is great when you are putting up decorations for the holidays.
Plus, it uses safe low voltage parts, so you can use it without worrying about safety while getting everything ready for the festive season.
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