A simple timer to count down in time ranges from 0 to 99 seconds or from 0 to 99 minutes. Thanks to the built-in high current relay contact and intuitive operation, the timer can be used to perform time functions in home automation and industrial automation.
• 2 operating modes: seconds and minutes.
• Time setting in the range of 1-99 in steps of 1.
• Output relay with contacts 230V AC / 8A.
• 2-digits LED display.
• Power supply: 12V DC.
• PCB dimensions: display: 27×48mm; motherboard: 48×58mm.
• Electronic Kit – soldering required.
Schematic of the timer is shown in Figure 1. Timer is supplied with DC voltage from 8-12 V. Rectifying diode D1 protects the circuit from reverse polarity. The supply voltage is stabilized by the integrated stabilizer U1. Timer is based on ATtiny261 microcontroller clocked with an internal RC generator. The operating status is displayed on a seven-segment display with a common anode. The 2-digit LED segments of the multiplexed LED display are connected via R5-R12 limit resistors to the PA0-PA7 pins of the microcontroller. The T1 and T2 transistors are used to supply LED digits. Transistors are switch on/turned off by the signals from PB3 and PB4 pins of the microcontroller. Three buttons S1-S3 allow you to enter settings and enable timer functions. Buttons are connected to pins PB0, PB1 and PB6. Active level is low. The NO and NC contacts of the relay are connected to the pins of the output connector.
Assembly and test
The assembly is easy and typical. At the start, you must install the smallest components and finish on the largest ones. After assembling, both plates should be connected using an angular goldpin connector. The assembled system works flawlessly when the power is turned on. For heavy load control, pay attention to the load on the relay contacts and the PCB paths. In order to improve the load capacity of the tracks, they can be soldered to thick copper wire.
The time unit (seconds or minutes) is selected by the MIN/SEK jumper. The S1 and S2 buttons increase or decrease the time. Use the S3 button to start the timer. Each press of S2 button increases and S1 button decreases the time value. To make the change faster, press and hold the corresponding button.
The set value is stored in the non-volatile memory. If the dot is lit at the unity number, the timer counts down the minutes. If the dot is off, then the timer counts the seconds. Blinking a dot indicates time running. When you start the timer, at any time by pressing S3 user can stop the timer. In this case the digits on the display will blink. The timer waits for a short press of the S3 key or its longer hold time, after which it will restart. Warning! Counting the time may be a little inaccurate, especially for minute work.
C1, C2: 100µF
D1, D2: 1N4007
U2: ATtiny261 with 20-pin IC socket
T1, T2: BC557
SV1: Goldpin connector 1×16pin
MIN/SEK: Goldpin connector 1×3pin+ jumper
ZAS, NO, NC: 2-pin terminal block connector