TIMER WITH RELAY
Price - Stock No 270This kit operates a relay for a specific time. The circuit can be triggereg by a high going pule or a low going pulse. As soon as it is triggered the LED illuminates and the relay energises, they remain in this state for a set time then turn off. The time is set by the trimpot and in the configuration shown the range is from approximatly 1.1s to 12.1s.
Operation and Testing
This kit uses the 555 timer in a one shot (monostable)
mode. View the Datasheet for the
When the button is not depressed, the 555's trigger is held high (12V) by the 10K resistor. When the button is pressed, the trigger goes low (0V) setting the timed circuit into action. The 555's output (pin 3) goes high, illuminating the LED and energising the relay. The 555's discharge (pin7) and threshold (pin6) now allow the 100uF capacitor to begin charging via the 10K resistor and 100K trimpot. When the voltage at pins 6&7 reach 8V (two thirds of 12V) the 555 sends it's output low, the LED and relay go off. The 100uF capacitor is discharged via pins 6&7 and the system is ready for the next trigger.
Note the diode across the relay, this is a good example of physics in reverse. When we energise the coil of the relay the current is turned into a magnetic field and changes the state of the relay. When we turn the relay off,the collapse of the magnetic field is turned back into electrical current by the coil in the relay. This current is in the reverse direction as the magnetic field is collapsing. This reverse current can damage the 555 and is bled off by the diode.
Troubleshooting, if necessary will involve careful checking of locations and polarity of components, in particular the diodes, the 555 and 100uF capacitor. Re-solder all joints and check to make sure you have not bridged across between any two adjacent component legs. The end of a broken hacksaw blade sharpened on an emery wheel is a good tool for cleaning between soldered joints.
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