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Tiny Timer Schematic Circuit Diagram

Revolutionizing Lighting Control: Crafting an Innovative Timer Switch

In the pursuit of creating an advanced lighting control unit, the author encountered a significant obstacle: the existing analog and mechanical timers lacked the precision and user-friendliness required. Undeterred, the author swiftly engineered a timer switch powered by a compact AVR controller, specifically the ATtiny2313. This cutting-edge device boasts remarkable accuracy, allowing seamless load switching with one-second precision. Its operational window spans from a mere second to an impressive duration of 99 hours, 59 minutes, and 59 seconds.

The ingenuity lies in the integration of a diminutive yet efficient LCD display, the HMC16223, measuring a mere 52 mm x 20 mm. This streamlined design enabled the prototype to snugly fit within a standard wall outlet box. The ATtiny2313, synchronized with a 4.9152 MHz crystal, generates an internal timebase precisely calibrated to one second. Facilitating user interaction, the LCD operates in 4-bit mode, with data input facilitated through press buttons cleverly utilizing the controller’s built-in pull-up resistors. Furthermore, the system ensures electrical isolation between the AC grid and the controller-LCD setup via a miniature transformer rated at 9 V and 1.5 W. This innovative timer switch marks a significant leap forward in lighting control technology.

Tiny Timer Schematic Circuit Diagram

For small switching loads (below 200 W) the power relay can be replaced by an all-electronic solid state relay (e.g. Sharp S202 S02).

AC power line voltage circuits are not for beginners and it’s vital to observe the relevant safety guidelines at all times!

You are recommended to divide the circuit between two separate boards: LCD, microcontroller and press buttons on one and transformer, rectifier and switching relay on the other.

Here’s how you use the timer switch: When the timer is running, the preset time period and time remaining are displayed on the readout:

  • PRESET 1:10:08
  • COUNT 0:09:59
  • An alternative format can be selected if you wish:
  • PRESET 1h 10m 8s
  • COUNT 0h 9m59s
  • The four function buttons are used as follows:
  • START: Start timer for the preset period
  • STOP: Stop timer, select menu for setting values and options
  • PLUS: Increment the selected value by 1
  • MINUS: Decrement the selected value by 1
  • The following values can be set:
  • Menu 1: SET HOURS 00
  • Menu 2: SET MINUTES 00
  • Menu 3: SET SECONDS 00
  • Menu 4: SET DISPMODE 0

Push buttons PLUS and MINUS alter the selected value and pressing them both simultaneously resets the value to zero.

A programmed controller is available from the Elektor shop under order code 091044-41 ( If you enjoy doing the programming yourself, the fuses of the ATtiny2313 should be set as follows:

EXT. byte: 0xFF – (brownout det. off, no CKDIV8)

HIGH byte: 0xDF – (ext. crystal > 3 MHz)

LOW byte: 0xFD – (64ms startup)

As usual, the hex and source code can be downloaded free from the Elektor website (


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