Many electronics circuits and projects for learning in simple ways.

The light meter circuit using a general diode

This is a simple light meter circuit using normal diode as a sensor and show amount of light by a any voltmeter.

The light can change the properties of many devices, example..
LDR as photo resistors can change resistance by light.
Photodiode- like LDR but sensitive better than LDR, because convert light into a electricity current.

Some times general photodiode may be expensive and hard to buy. When the light to a silicon diode will have very tiny current So we can easily use it instead.

Light meter Using a Diode

The 1N914 is popular number, cheapest, small and clear body. In reverse bias form, when light to it will cause a photo current leak through to a P-N junction.

Measuring the voltage of the diode when light hits it
Measuring the voltage of the diode when the light hits it.


How to choose the diode.

+ We need the high sensitivity diode only.
+ Shine a flashlight or sunlight on the 1N914 diode.
+ In Figure 1, measure voltage by a digital voltmeter only, because the photo current is very very low, an analog meter cannot read.
+ If more light, will makes more diode current.

Current to voltage converter

Current to voltage converter using op-amp
Current to voltage converter using an op-amp

Then, apply it to a current to voltage converter circuit. In Figure 2. This circuit use the CA3140 OP-AMP on an inverting mode.

The resistor-R1 set a circuit’s gain, and the output voltage will rise if we increase R1’s resistance. Thus will get the formula is :

Vout = Id.R1

In this formula: Id = the diode current

The resistor R1 = 1M, but we may try to use 100 K to 10M, to adjust the proper gain ratio.

The voltage divider circuit include R2, R3 and VR1, to compare the circuit with the light meter calibration standard

How to setting
May use A 40-watt incandescent bulbs without Reflector. Placed away from the light meter is 50cm.(which will be a brightness of 150 lux approx.) Then adjust VR1 to read the Vout is the number 150 (1.5V). So, can use this to directly measure others brightness. If cannot read is 150, may be change the new R1’s resistor to lower or higher up.

We can use the analog voltmeter is cheap, or digital is easy to read and high correct than the analog types. Also, a vu-meter as galvanometer is cheapest but smallest.

The 1N914 diode have Parabolic reflector shape to increase the sensitivity and good direction. Be used aluminum foil as a reflective.

Complete Light meter circuit idea using diode
Complete Light meter circuit idea using a diode

Figure 3 The full circuit diagram

In Figure 3. If need to measure a lower brightness, may add offset adjusting of op-amp and change the power supply is positive-negative types. If single power supply, can use voltage 6-30V, but dual types is 3-15V.

This Post Has 2 Comments

  1. You can do the same thing with an LED, but the LED will only respond to a narrow frequency (red will respond to red light, green will respond to green light, white won’t do anything because they’re actually a blue LED with phosphorus and doesn’t respond well), but will put out a higher voltage.

  2. hi,jammit.
    Thanks for your feedback.
    and Thanks for you suggestion.

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