# Transistor As Switch

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#### azar_pisces

##### New Member
Hello
I was designing a light operated switch which uses optocoupler for isolation. in fact i am stuck at the calculations of transister which drives the opto. I need help to find out values for resistors R3 R4 and R7.Supply voltage is 5v

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#### Monkey

##### New Member
R7 = 4.7Kohm -> 1mA BE current
as the transistor is saturated, Vce=0.2V
Current through the LEDS should be around 10mA, thus R3=R4= 470ohm.

#### azar_pisces

##### New Member
Thankyou for ur help.
Another thing I wanted to know is, will these values drive the transistor somehow deep into saturation?
I shall be happy if you may provide some maths formulae for calculations.
thankyou once again

#### ccurtis

##### Well-Known Member
Thankyou for ur help.
Another thing I wanted to know is, will these values drive the transistor somehow deep into saturation?
I shall be happy if you may provide some maths formulae for calculations.
thankyou once again
1. Determine the required collector current (Ic) when the transistor is in the ON state (the emitter and collector are short circuited).

2. From the transistor data sheet, determine the minimum DC gain (hFE) of the transistor at the value of collector current determined in step 1.

3. Calculate the required base current (Ib)

Ib=Ic/hFE

For power transistors, it may be wise to double or triple the calculated value of Ib because hFE drops to low values at very high currents and the data sheet does not show all values of collector current.

4. Calculate the base resistor value (Rb) from the voltage applied to the base resistor (Vb) and from Ib, calculated in step 3.

Rb=(Vb-0.7)/Ib

5. To drive the transistor deeper into saturation, reduce the value of Rb, taking care not to overload the source that provides Ib. However, driving deeper into saturation wastes power in Rb without gaining much reduction in the collector-emitter saturation voltage.

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