Category Archives: Measuring

Colour Sensor

Colour sensor is an interesting project for hobbyists. The circuit can sense eight colours, i.e. blue, green and
red (primary colours); magenta, yellow and cyan (secondary colours); and black and white. The circuit is based
on the fundamentals of optics and digital electronics. The object whose colour is required to be detected
should be placed in front of the system. The light rays reflected from the object will fall on the three convex
lenses which are fixed in front of the three LDRs. The convex lenses are used to converge light rays. This helps
to increase the sensitivity of LDRs. Blue, green and red glass plates (filters) are fixed in front of LDR1, LDR2
and LDR3 respectively. When reflected light rays from the object fall on the gadget, the coloured filter glass
plates determine which of the LDRs would get triggered. The circuit makes use of only ‘AND’ gates and ‘NOT’

When a primary coloured light ray falls on the system, the glass plate corresponding to that primary colour will
allow that specific light to pass through. But the other two glass plates will not allow any light to pass through.
Thus only one LDR will get triggered and the gate output corresponding to that LDR will become logic 1 to
indicate which colour it is. Similarly, when a secondary coloured light ray falls on the system, the two primary
glass plates corres- ponding to the mixed colour will allow that light to pass through while the remaining one will
not allow any light ray to pass through it. As a result two of the LDRs get triggered and the gate output
corresponding to these will become logic 1 and indicate which colour it is.

When all the LDRs get triggered or remain untriggered, you will observe white and black light indications
respectively. Following points may be carefully noted :

1. Potmeters VR1, VR2 and VR3 may be used to adjust the sensitivity of the LDRs.
2. Common ends of the LDRs should be connected to positive supply.
3. Use good quality light filters.

The LDR is mounded in a tube, behind a lens, and aimed at the object. The coloured glass filter should be
fixed in front of the LDR as shown in the figure. Make three of that kind and fix them in a suitable case.
Adjustments are critical and the gadget performance would depend upon its proper fabrication and use of
correct filters as well as light conditions.

Analog Milliamp Meter Used as Voltmeter

A milliamp meter can be used as a volt meter by adding a series resistance. The resistance needed is the full scale voltage reading divided by the full scale current of the meter movement. So, if you have a 1 milliamp meter and you want to read 0-10 volts you will need a total resistance of 10/.001 = 10K ohms. The meter movement itself will have a small resistance which will be part of the total 10K resistance, but it is usually low enough to ignore. The meter in the example below has a resistance of 86 ohms so the true resistor value needed would be 10K-86 or 9914 ohms. But using a 10K standard value will be within 1% so we can ignore the 86 ohms. For a full scale reading of 1 volt, the meter resistnace would be more significant since it would be about 8% of the total 1K needed, so you would probably want to use a 914 ohm resistor, or 910 standard value. The milliamp meter can also be used to measure higher currents by adding a parallel resistance. The meter resistance now becomes very significant since to increase the range by a factor of ten, we need to bypass 9/10 of the total current with the parallel resistor. So, to convert the 1 milliamp meter to a 10 milliamp meter, we will need a parallel resistor of 86/9 = 9.56 ohms.
Copyright 2006 Bill Bowden

AC Line Current Detector

This circuit will detect AC line currents of about 250 mA or more without making any electrical connections to the line. Current is detected by passing one of the AC lines through an inductive pickup (L1) made with a 1 inch diameter U-bolt wound with 800 turns of #30 – #35 magnet wire. The pickup could be made from other iron type rings or transformer cores that allows enough space to pass one of the AC lines through the center. Only one of the current carrying lines, either the line or the neutral should be put through the center of the pickup to avoid the fields cancelling. I tested the circuit using a 2 wire extension cord which I had separated the twin wires a small distance with an exacto knife to allow the U-bolt to encircle only one wire.

The magnetic pickup (U-bolt) produces about 4 millivolts peak for a AC line current of 250 mA, or AC load of around 30 watts. The signal from the pickup is raised about 200 times at the output of the op-amp pin 1 which is then peak detected by the capacitor and diode connected to pin 1. The second op-amp is used as a comparator which detects a voltage rise greater than the diode drop. The minimum signal needed to cause the comparator stage output to switch positive is around 800 mV peak which corresponds to about a 30 watt load on the AC line. The output 1458 op-amp will only swing within a couple volts of ground so a voltage divider (1K/470) is used to reduce the no-signal voltage to about 0.7 volts. An additional diode is added in series with the transistor base to ensure it turns off when the op-amp voltage is 2 volts. You may get a little bit of relay chatter if the AC load is close to the switching point so a larger load of 50 watts or more is recommended. The sensitivity could be increased by adding more turns to the pickup.

Copyright 2006 Bill Bowden