BASIC 
ELECTRONICS COURSE 
Page 46 INDEX

THE COIL  (. . . continued from previous page)
The coil can be connected to the "analogue-to-digital" circuit we covered on page43.

This requires an input waveform of about 100-300mV to change the state of the first transistor. The circuit sits with the first transistor turned ON via the base-bias resistor and if we assume the base is 0.7v, we need to lower the voltage to below 0.55v to turn the transistor OFF. 
The output of the coil needs to be at least 150mV to do this, and providing the coil can deliver this voltage, the circuit above can be used. 
The only way you can decide if the circuit can be employed, is to connect the coil (the magnetic transducer) and monitor the results. 
The greatest output from a coil is obtained when the coil has the maximum number of turns and the magnet passes through the centre of the coil with the greatest velocity. 
To double-check the suitability of the circuit, the magnet is passed through the coil at a slightly slower speed and if it still operates, the circuit can be used. 

It is necessary to place a capacitor between the coil and the input of the circuit. This is to allow the the biasing resistor to put 0.7v on the base. If the circuit is connected directly to the coil, the voltage on the base may be lower than that necessary to turn the transistor ON (0.55v) (the actual voltage on the base will depend on the resistance of the coil). 

The output (line X) can be connected directly to a digital gate or microcontroller input-line.  

to see the circuit above in action. Remember: A North pole entering a coil produces a voltage of same polarity as a South pole exiting a coil. Notes:  1: Each graph has a different voltage scale.  2: A magnet must pass a coil quickly so that the pulse produced  will pass through the 22n capacitor.  3: Note the quick operation of the second transistor - from one state to the other - this is called "DIGITAL OPERATION."

THE PHOTO TRANSISTOR
The Photo transistor is an ordinary transistor in a clear plastic case. Almost any transistor will respond to light and that is why they have to be in a light-tight case. The silicon material making up the junction of the transistor changes resistance when it sees the energy from a light-source and this change is detected between the collector and emitter leads. 
Sometimes as second transistor is also mounted inside the case and it amplifies the change in resistance to produce a more-sensitive device. The combination is called a PHOTO-DARLINGTON TRANSISTOR.  
Two less-sensitive devices are also available. They are a PHOTO-DIODE and PHOTO-RESISTOR (also called a LIGHT-DEPENDENT RESISTOR - LDR). 
Some devices detect visible light while others detect infra-red. 
Getting a photo-detecting circuit to work requires a little bit of experimentation. 
You need to determine if the device is sensitive enough for the application and then adjust the circuit for the LIGHT and DARK conditions. 

The circuit above can be adjusted so that almost any light-level can be detected. When light is detected by the MEL-12 photo-transistor, its resistance DECREASES. This means the voltage-level sitting on the left-side of the 10u electrolytic is brought down towards the 0v rail. This causes the right-side of the electro to fall ALSO. The result is the voltage-level on the base of the first transistor is REDUCED. This turns the transistor OFF. 
The 50k mini-trim pot changes the gain of the photo-transistor and makes it more-sensitive as the resistance of the pot is decreased. The result is the voltage-fall on the left-side of the 10u electrolytic is greater for any given change in light and this means a very small change in light-level will cause the circuit to operate. 
The circuit will only detect an increase in light. When the light-source is removed, the circuit does not respond. 
The circuit below is designed to detect when the light-source turns OFF:

In the following circuit, the A-to-D stage and 4011 NAND gate have been replaced by a Schmitt Trigger. For details on the Schmitt Trigger click HERE.
The MEL-12 Darlington Photo-transistor is connected directly to the input of the Schmitt gate and the circuit can be adjusted via the 50k mini-trim pot. 

When the MEL-12 receives light, the  voltage on the input of the Schmitt gate drops from a HIGH value to a LOW value. For the circuit to work, the LOW value must be 33% of rail voltage or less. 
When a light-source is removed from a MEL-12, the voltage on the input of the Schmitt gate rises to a HIGH value. For the gate to change state, the voltage must rise to 66% of rail voltage or higher. 
The animation below shows how the mini-trim pot changes the gain of the photo-transistor.

If the change in light intensity is not sufficient to operate the Schmitt gate, you can add the A-to-D stage described above. You will need to include the 10u electrolytic for the A-to-D amplifier to operate. 

NEXT