Voltage, Current and Resistance

Voltage, Current and Resistance

The relationship between Voltage, Current and Resistance forms the basis of Ohm’s law which in a linear circuit states that if we increase the voltage, the current goes up and if we increase the resistance, the current goes down. Then we can see that current flow around a circuit is directly proportional ( ∝ ) to voltage, ( V↑ causes I↑ ) but inversely proportional ( 1/∝ ) to resistance as, ( R↑ causes I↓ ).

• Voltage or potential difference is the measure of potential energy between two points in a circuit and is commonly referred to as its “ volt drop ”.
• When a voltage source is connected to a closed loop circuit the voltage will produce a current flowing around the circuit.
• In DC voltage sources the symbols +ve (positive) and -ve (negative) are used to denote the polarity of the voltage supply.
• Voltage is measured in “ Volts ” and has the symbol “ V ” for voltage or “ E ” for energy.
• Current flow is a combination of electron flow and hole flow through a circuit.
• Current is the continuous and uniform flow of charge around the circuit and is measured in “ Amperes ” or “ Amps ” and has the symbol “ I ”.
• Resistance is the opposition to current flowing around a circuit.
• Low values of resistance implies a conductor and high values of resistance implies an insulator.
• Resistance is measured in “ Ohms ” and has the Greek symbol “ Ω ” or the letter “ R ”.

LDR: Light Dependent Resistor

Light dependent resistor(LDR), also called as Cadmium Sulphide (CdS) cell or photoresistor is a resistor whose value depends upon the incident light intensity. In darkness, the resistanceof LDR becomes high and in light, the resistance gets low.

Dark Sensor Using 555

The following circuit works as a dark sensor. If the light falling on LDR is blocked, it switches on the LED D1. It has been configured in monostable mode of 555 timer which means that the circuit switches on the LED D1 for a specific time. The switching time is determined by the combination of resistance R1 and capacitor C1.

In this circuit, time (T) = 1.1xR1xC1 = 1.1x47x103x100x10-6= 5.17sec.

Touch Sensor using 555
We can make the previous dark sensor circuit work as a touch sensor by making small changes in the schematic. We just need to remove the LDR, put a small wire and connect it to the coupling capacitor. Note that the electrode wire connected to capacitor C2 should be around 2-5cm long. As we touch on the wire, the circuit triggers the LED for a specific time as determined by the resistor R1 and capacitor C1.  You can connect the wire to a small metallic touch plate using an alligator clip.