[ TERC VHS-1 Sun Photometer ] [ Manual TOC ]
Figure 1 (to the right) is an electronic circuit diagram of the TERC VHS-1. The triangle marked "741" is an operational amplifier. In operation, light from the Sun causes the LED detector to generate a tiny electrical current much like a solar cell does. This current goes to the operational amplifier, which is connected so that the LED current is transformed to a voltage and boosted by the amount of the feedback resistance in ohms between the input (pin 2) and output (pin 6) of the amplifier. So if the feedback resistor has a resistance of 3,900,000 ohms, the amplifier boosts the signal from the LED by a factor of 3,900,000. The voltage from the amplifier is measured with a digital voltmeter.