AUTOMATICNIGHTLIGHT

BUILD YOUR OWN NIGHTLIGHT CIRCUITKit-ABRA-102

• Masterthedarknesswithyourvery

ownAutomaticNightlightKit• Adjustyournightlighttoturnon

whenyouconsideritdark.• Controlthebrightness.• LearnhowaLightDependentResistorworks.

• LearnhowaTransistorworks.• LearnhowtouseaLEDlightwith

currentlimitingresistor.• Batteryoperated

forsafety.

• DualwhiteLED’s.

Kit Description:This NIGHTLIGHT KIT turns on a pair of LED lights as it gets dark.The potentiometer allows setting the point of darkness and the brightness of the LED’s.

Circuit Description: The Light Dependant Resistor (LDR1) resistance varies with the amount of light falling on it.LDR1 resistance varies from approximately 1 Meg Ohm in darkness to 100 Ohm in full light.This change in resistance in used to control the amount of current flowing in transistor Q1 and the brightness of the LED’s. The potentiometer (P1) is adjusted in full light so that the LED’s are just turned off. The LED’s brightness will increase as the light dims. When the light is completely off the LED’s will light up to maximum brightness. There are multiple lessons to be learned from this simple circuit.• WhydoweuseadiodeD1?• WhatisaLightDependentResistor?• HowdoestransistorQ1workinthecircuit?• HowdoyoudeterminethecurrentlimitingresistorforLED’s? Diode D1 is used to protect the circuit by only allowing the positive (+) voltage to reach the circuit. A diode can only pass current one way. This prevents any possible damage from a reverse hookup of the battery connector or power supply adaptor.The Light Dependent Resistor is made of a compound of Cadmium Sulfide (CdS) that changes resistance with the amount of light falling on it. LDR1 is approximately 1 mega ohm (1M) in the dark and 100 ohms in the light.Transistor Q1 acts like a current valve. It has three connections an Emitter (E), Base (B) and Collector (C). It controls the amount of current passing between its Emitter (E) and Collector (C) depending on the amount of current flowing into the Base (B).The small current flowing into the Base is amplified to pass a much larger current between the Emitter and the Collector. Potentiometer (P1) controls the voltage appearing at the base of Q1 (and current flowing into the base) by forming a voltage divider with LDR1. As LDR1 resistance increases with increased darkness the voltage appearing at the junction of P1 and LDR1 increases and in turn increases the current flow into the Base. This increase in current is amplified by Q1 and results in a much larger increase in current flowing through the LED’s.The voltage at the base of Q1 is determined by the formula: V(base) = V(battery) x LDR1/R1+LDR1Ex.: When dark LDR1=1M ohm and V(base) = 9.0x1M/1M+22K or 8.8 volts When light LDR1=100 ohm and V(base) = 9.0x100/22K+100 or 0.04 voltsWhen LDR1 is completely dark with 8.8 volts on the base, the transistor is fully turned on and acts as a switch with R2 limiting the maximum current into the LED’s. Any voltage can be used to drive LED’s with the appropriate current limiting resistor.LED’s are usually specified to operate with a nominal 20mA (0.020A) current. The value of a current limiting resistor can be determined by Ohm’s Law: R = E/I where R is the required resistance, E the voltage of the supply and I the max current allowed for the LED. Note that the forward voltage drop (VF) across the LED’s has to be subtracted from the total available voltage from the supply to correctly calculate the resistance. So the formula becomes R (limiting) = E-VF/I(max). For white LED’s VF is approximately 3.0V For a current limit of 20mA, the calculation for R2 would be: R2 = 9.0-3.0/.020 = 300 ohms.In this kit a value of 470 ohms was chosen to limit the current to 12mA to prolong battery life.

Tools Required: •Soldering Iron •Solder •Diagonal Cutters •9V Battery

Assembly step by step:1. Install Resistors R1, R2, as shown in the PCB LAYOUT using their color codes to

identify them.2. The leads can go in either way. Solder the resistors in place and trim the leads with the diagonal cutters.3. Install Diode D1, making sure the white line on the diode matches the PCB Layout. Solder the diode in place and trim the leads. 4. Install LED1 and LED2 with the short lead (CATHODE) towards the edge of the PCB

and the short lead (ANODE) towards the inside of the PCB. Solder LED1 and LED2 in place and trim the leads.5. Install Q1 with flat side towards the outside of the PCB as shown in the PCB LAYOUT. Solder Q1 in place and trim the leads.6. Install LDR1 as shown on the PCB LAYOUT. The leads can go in either way. Solder LDR1 in place and trim the leads.7. Install P1 as shown in the PCB LAYOUT. The leads can go in either way. Solder P1 in place and trim the leads.8. Solder the red Battery Clip wire to the +5V point on the PCB (at D1). Solder the black Battery Clip wire to the GND point on the PCB (LDR1, LED1 junction).9. Before connecting a 9 volt battery review your work as per the Assembly Check Out.

PCB board layout Schematics;(copper traces as seen through the board);

General Assembly Guidelines:Good soldering technique is the most important factor in successfully assembling your kit.A small pencil type soldering iron 25-40 Watts is recommended.The tip of the iron should be kept clean at all times by wiping it often on a damp sponge or cloth.Touch the component lead and the solder pad junction with the soldering iron tip.Apply solder to the junction. It should not take more than 1-2 seconds for the solder to melt. A good solder joint will appear shiny and conform to the lead.Refer to the Abra website at www.abra-electronics.com using the part number if you can-not identify a part in the part list.Parts list: Item ABRA Part# Description Quantity

D1 1N4001 Diode 1P1 3386-F-104 Potentiometer 100K ohm 1Q1 2N3904 Transistor NPN 1R1 R1/4-22K Resistor (red,red,orange) 1R2 R1/4-470 Resistor (yellow,violet,brown) 1LDR1 PHOTO-310 Light Dependent Resistor 1LED1,LED2 LED-5W 5mm LED white 29V CLIP 29-130 9V Battery Clip 1PCB Board1 Nightlight PCB 1

www.abra-electronics.com U.STollFree:1-800-717-2272

sales@abra-electronics.com Can.TollFree:1-800-361-5237MADE IN CANADA

Assembly Check out: • Before installing the 9 volt battery it is highly recommended to inspect the PCB carefully.• Check for proper placement of components.• Check that the diodes and transistor are installed in the correct directions.• Check that all connections are soldered with a shiny appearance. • Redo any solder connection that is dull looking or in a ball.• Check that there are no solder bridges touching adjacent connections together.

Solder Profiles:

Operation: 1. Install a 9V battery into the battery clip. With lights on adjust P1 until the LED’s light up and then go back until they are just barely on. Turn the lights off and the LED’s should light up brightly.2. Adjust P1 to get the desired brightness in the dark.

Suggestions:• Replace the 9 Volt Battery with an AC-DC adaptor such as our DC-950 to avoid hav-

ing to change batteries.

Troubleshooting:- If the LED’s do not turn on throughout the range of adjustment of P1, repeat the Assembly Checkout. - If the Assembly Checkout does not reveal any faults, replace the battery.