This construction uses a light‑dependent resistor (LDR) which changes its resistance according to the amount of light falling onto its surface. The LDR has a low resistance in daylight but, as the light level drops at dusk, its resistance increases which causes the transistor, TR1, to switch on the light‑emitting diode, D1. At dawn, the light level increases and the transistor switches off D1.
Resistor R1 is connected to D1 to reduce the amount of current flowing through it. Too much current can cause a light‑emitting diode to burn out.
This project requires one PP3 battery (not supplied in this kit). Very little energy is consumed during daylight and so the lifetime of the battery depends on the length of time for which the circuit operates in darkness. In a test of this circuit, a Duracell Plus Power battery was still producing a good output of light after 120 hours of continuous use with the LDR covered with black tape to simulate darkness.
To construct this project you will need to use the following tools (not supplied):
Assembly Instructions for Project 9: Darkness Detector
Please refer to the wiring diagram and read through these instructions very carefully before starting assembly.
1. Identify resistor R1 which has bands of yellow, violet and brown. It does not matter which way around a resistor is connected in a circuit. Bend the leads of R1 so that they can be pushed through holes A2 and E2. Spread the ends of the leads of the resistor slightly apart and carefully solder. Avoid solder splashing across the copper tracks where you have soldered the resistor. Trim excess wire from the leads of the resistor using wire cutters.
2. Identify the positive and negative leads of the light‑emitting diode, D1. A light-emitting diode will not work if it is connected the wrong way around and so correct identification of the leads is important! For this type of l.e.d., the negative lead is shorter and it is next to the flattened edge on the l.e.d. capsule (see picture on wiring diagram above).
3. Push the positive lead of D1, without bending it, through hole E6 and, using pliers, bend the negative lead so that it fits through hole C6. Spread the ends of the leads slightly apart and carefully solder. Avoid solder splashing across the copper tracks where you have soldered the l.e.d. Trim excess wire from the leads of the l.e.d.
4. It is essential that the transistor, TR1, is correctly connected to the other components. Identify the three leads of TR1. The emitter lead is nearest to the tag on the metal canister of the transistor. The base lead and the collector lead are also shown in the picture below:
Skill level: * * INTERMEDIATE
Project constructed on track board.
This circuit contains a light‑emitting diode which switches on at dusk. The l.e.d. switches off as the light level increases at dawn. The circuit diagram is shown below.Type your paragraph here.
Twist the free end of one of the cables attached to S1 around the terminal pin at A19 and solder them together. Attach the other free cable to terminal A22 and solder. Make sure that a stray wire or excess solder does not bridge the gap between the two terminal pins.
10. Twist the red lead from the battery clip around the terminal pin in position A25 and solder them together. Twist the black lead from the battery clip around the terminal pin in position I25 and solder them together.
11. Assembly is now complete. Carefully compare your circuit with the wiring diagram and the pictures above. Check, again, that there are no solder splashes across adjacent tracks and that the track is completely broken in the correct position.
Connect a new PP3 9 volt battery to the battery clip and switch on. Test your circuit by reducing the amount of light falling on the LDR by placing your finger over it or by moving the circuit to a darkened room. The light‑emitting diode should switch on. Then, increase the amount of light and the l.e.d. should switch off.
Wait for darkness to fall to properly test the operation of your circuit!
9. Identify the two cables for the slide switch, S1. Push the end of one of the cables through the hole in the central tag of the switch, gently twist together and solder. Push the end of the second cable through the hole of either one of the remaining tags, twist and solder. Make sure that solder does not bridge the gap between these two connections on the switch. (The third tag on the switch is left unconnected.) Your switch should now look like that shown in the picture below:
close-up of leads of transistor TR1
close-up of track break
at A21 and terminal pins
showing soldered joints
switch S1 and
5. Resistor R2 has bands of blue, grey and yellow. Using pliers, bend the leads of R2 to fit through holes A14 and G14. Spread the ends of the leads of R2 slightly apart and carefully solder. Trim excess wire from the leads of R2.
6. Identify the light‑dependent resistor. It does not matter which way around it is connected in a circuit. Push the leads of the LDR through holes G19 and I19. Spread the ends of the leads slightly apart and carefully solder. Trim excess wire from the leads of the LDR.
7. Push terminal pins from the track surface through to the top surface in positions A25, I25, A19 and A22. Terminal pins are a tight fit and pliers are usually needed to squeeze them through the holes. (Terminal pins have a habit of jumping off the table and are never found again - two spare terminal pins are provided in the kit.)
8. It is much easier to break the copper track at position A21 before attempting to solder the terminal pins to the track surface. A copper track can be broken by carefully scraping the copper away with a sharp craft knife or, more easily, by pressing a 3 mm drill bit (not supplied in this kit) into a hole on the track surface and gently twisting the drill until all of the copper track at that position has been removed. The pictures below show the track break and terminal pins on track A before and after soldering:
close-up of top
D1, R1 and TR1
PROJECT 9: DARKNESS DETECTOR
Think SAFETY. Act SAFELY.
You're important . . . so also read through the safety guidance sheet provided with the kit.
close-up of track break
and soldered terminal pins
Using pliers, bend the leads so that the emitter lead fits through hole I10, the base lead fits through G12 and the collector lead fits through C10. Spread the ends of the leads of TR1 slightly apart and carefully solder. Trim excess wire from the leads of TR1. The pictures below show this stage of construction:
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