# Circuit 38 – 555 Off for a period

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```					Circuit 38 – 555 (Off for a period).
Function
When this circuit is triggered, it will remain off for a certain time. This effect is particularly
useful when arming an alarm switch, as it gives the owner time to vacate the house before alarm
is armed.
Circuit diagram

SW 1

R2    R3 7 8        4                         9V

2                             LED
555   3
6
R1
VR 1              5     1

PTM
+
C1                                      0V

Components
SW1 = Toggle Switch
Switch = (PTM)
R1 = 470 ohm
R2 = 33k ohm
R3 = 1k ohm
VR1 = 100k ohm
C1 = 2200F
IC1 = 555 Timer
5mm Standard LED

Operation.
555 timer is configured as a monostable circuit.
This means when the IC is triggered, the output (pin 3) will go high (9V) for a pre-determined
time i.e. the LED is unlit. Output pin 3 will then return low, (0V), and remain in this condition
until retriggered.
How circuit is triggered
Consider PTM switch is open, the voltage at bottom of R2 is 9V (connection also to pin 2).
When PTM switch is closed the voltage on pin 2 will drop to 0V, and if immediately released the
voltage will again jump to 9V i.e. a negative pulse has been produced. This negative pulse
applied to pin 2, triggers the IC, and the output (pin 3) goes high for a period that can be set by
theVR1, C1 combination
Formula for time high =1.1 x VR1 x CI seconds.
The value of R3 is very small, so can be ignored.
In this case if VR1 is set at 100k ohms, time high would equal:
Time high = 1.1 x 100,000 x 0.002200 SECS seconds = 242 seconds.
When this circuit is triggered output at pin 3 goes to 9V.
As the LED + R1 are connected from pin 3 to the top rail and as both points are at 9V, no current
can flow in the LED, it is unlit for 242 seconds.
When output pin goes to 0V after specified time, rail voltage is 9V so current can now flow from
the top rail through R1, the LED and into the chip. The LED is lit.
.
Calculation for Value of R1
It is most important an LED must have no more than 2 volts dropped across it. Also the current
flowing through should not exceed 20 mA. When the voltage at pin 3 is 0V between the top rail
and the output (pin 3) is 9V, so 7V must be dropped across R1.
With 7V dropped across it, and the maximum permissible current of 20 mA flowing through the
LED, the size of resistor can be calculated using Ohms Law:

7
R1           350ohms
0.020

Note
In electronic circuits it is not good practice to have components operating at their upper limits, so
a resistor value of 390 or 470 ohms should be used for R1.
Remember the larger the value of R1, the less current is drawn from the battery, so the battery
will last longer at the expense of the LED being a little dimmer.

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Description: Circuit 38 – 555 (Off for a period)