Week 7(Discription Of The Project)

Circuit 1. Trigger circuit diagram of the versatile optical trigger

MEL 12 phototransistor, TR1 is a very sensitive device
incorporating an amplifying circuit in the form of a Darlington pair.
In this circuit, we do not make connection to the base in the trigger circuit
because the light energy falling on the phototransistor generate the equivalent of a small base current.
This is amplifier within the device to produce a collector-emitter current
in the region of 3mA in bright light.
When the light is shining on the phototransistor,
the current flow’s result through resistor R1 and level control VR1 is series.
If the light level increase, the current through phototransistor TR1 increase.
The voltage across R1 and VR1 increase and the voltage at pin 2 falls.
The fall in voltage at the inverting input (-) of an op-amp produce a rise in voltage at the output.
The op-amp is connected as a comparator,
with no negative feedback,
so it full open loop gain of around 200,000 comes into play.

Circuit 2. Circuit diagram for the differentiator circuit

The differentiator circuit diagram is a based on a second op-amp, IC2.
It takes its input from the trigger circuit.
Input A is from the junction of the voltage divider resistor R2/R3,
which sits at half the supply voltage.
Input B comes from the output junction of the trigger op.amp, IC1
and coupled to the inverter input (pin 2) of IC2 through C2.
The output of this op-amp is then used to drive TR2
and switch the load and l.e.d on or off.

From this circuit,

An increase in input voltage produce a fall in output voltage

Then, output is proportional to R and C. 

so, we can adjust the sensitivity by adjusting VR2

An Output is proportional to the rate of change of input voltage. 

This mean that even a very short and small input pulse 

can produce a high output pulse provide that the rate of change is high.

The third point above makes the circuit a little too sensitive to small “spikes” 

on the signal from IC1. 

Therefore, 

capacitor C1 is connected across the input to eliminate the effect of such spikes. 

When the light level on the sensor rises quickly, the output of IC2 falls. 

A low pulse passes across capacitor C3 to the input of the flip-flop IC3b/IC3c.
This type of flip-flop, built from two NAND gates, is stable if both its input are high.
Resistor R8 and R9 provide for this.

However,
a negative pulse arriving by way of C3 will briefly make pin 6 low
and so set the flip-flop, its output at pin 4 goes high.
The rising output is used to switch the transistor TR2 in the trigger circuit(fig 1).
The load is energized and D1 comes on.
The flip-flop is energized and D1 comes on.
The flip-flop is reset by briefly pressing push switch S1.