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 = 2200F 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.