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Circuit Notebook

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PICAXE refrigerator timer for cafes

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This timer was designed to temporarily switch a cafe refrigerator off, to reduce background noise when a meeting was being convened in the cafe area. It provides automatic switch-on after an adjustable period of at least two hours. This avoids the refrigerator being inadvertently left off.

With the current software, the timer has a settable range of 10-180 minutes in 10-minute increments but with some appropriate software tweaks,
it has a capability of milliseconds to about 49 days.

With change-over contacts on the relay, the timer could switch the load on or off for the timing period, for other applications.

The timer is based on a PICAXE-
08M microcontroller. It is toggled on or off whenever pushbutton S1 is pressed. LED1 and the relay are on when the pin 3 output is high during timing mode. LED2 flashes when the pin 5 output is high during timing and gives an indication of the time delay remaining by varying the mark-to-space ratio of the flash.

If the LED flash is long, the time delay has only just started; if the flash is short, the time is almost up.

Q1 is an NPN transistor that switches the load via relay RLY1.
Diode D2 protects against relay coil back-EMF and diode D1 provides power supply reverse polarity protection.

The relay should have contacts rated for 250VAC and 30A, to cope with the surge current of the induction motor in the refrigerator. Suitable units would be a Jaycar SY-4040 chassis-mount type with SPST contacts or Altronics S-4211 chassis-mount with SPDT contacts.

Pins 2 & 7 provide the standard
serial programming interface. The time delay can be preset within specific parameters via a special set-up mode when the circuit is powered up or to a wide range of delay times by software changes. LED2 flashes once for every 10 minutes of time delay set, whenever the unit is powered up. For example, six flashes indicates 60 minutes.

Set-up mode is obtained by holding down pushbutton switch S1 until a series of rapid flashes from LED2 is observed. The time delay setting is then incremented by 10 minutes for each subsequent press of S1. As during start up, indication of the time selected is provided by LED2 flashing once for every 10 minutes of time delay.

When the setting of 180 minutes (18 flashes) is exceeded, the delay rolls over to 10 minutes (one flash). The delay is written to EEPROM so that the setting is retained even with power disconnected. The set-up mode is completed by powering off, waiting a few seconds (for the power supply capacitors to discharge) and then powering on again.

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