Timer circuit

Posted on Feb 5, 2014

The schematic diagram is quite straight forward. SW1 is a pushbutton used for starting the timer. The oputputs are set logic level 0 when the timer is not triggered, and to logic level 1 when the timer triggers. The relay driver transistor TR1, which receives its base current from Nutchip output OUT1 via the resistor R1, supplies enough current to energize the relay.

Timer circuit
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The diode D1, connected in parallel with relay`s coil, safeguards the transistor against the high voltage that builds up on the coil when it is energized or released. The relay is a 5V coil type, . and it must be choosen accordingly to the load you want to drive (e. g. AC or DC load). Ask your electrician for a suitable model, and always keep an generous safety margin (usually 50% or more) in excess the maximum specs declared by the manufacturer. For example, it is safer not to surpass 500W for a relay specified for continuous 1000W load maximum by the manufacturer. The schematic shows a ceramic resonator connected to Nutchip pins 4 and 5. This kind of clock source ensures a timing accuracy usually better than 1%, which is suitable for most uses. If you are looking for an even more accurate timer (e. g. for daily or weekly timers), a better choice is the quartz clock oscillator as shown in the base circuit collection. The same page shows alternate reset circuits that can be used when maximum reliability is required (this circuit uses a simplified RESET, the pin connects to the positive rail through an RC network made from R2 and C2). The circuit is simple and can be assembled in just an hour if you have a printed circuit board (PCB). Alternatively you can use a prototype board, in that case it requires more time and patience to be completed. Alwai start from smaller parts, leaving the bigger parts for later in order to...

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