School of Physics and Astronomy How to make a dark detecting torch Chris Staddon – Senior Experimental Officer & Outreach Officer Michael Forrester – Student Helper Acknowledgements: Modified from:

School of Physics and Astronomy Potential Divider Circuit The Potential (Voltage) is shared between the resistances LDR Light - low resistance Dark - high resistance

School of Physics and Astronomy The Mid-point needs 1.8 V to switch on 0.6 V transistor +1.2 V LED ratio of voltages = ratio of resistances so 1.8 / 3.0 = LDR / ( 2k4+LDR ) How it Works

School of Physics and Astronomy Components 1 x Prepared four-way terminal block (Choc block) 1 x Prepared battery clip 1 x LED (Light Emitting Diode – red) 1 x Transistor 1 x Resistor 2 x Prepared wires (red and black Battery supplied separately

School of Physics and Astronomy Now build your torch !