Count down timer based on PIC16F628A and OLED display module. Included C code and circuit diagram. Time span is 99 minutes. Setting the time is by the Rotary Encoder, for increasing or decreasing the time.
When the time reaches zero the buzzer sounds for 30 seconds. To stop the buzzer turn the Rotary Encoder.
22nF caps are for switch bounce.
OLED module is 0.96" 128x64, 4 pins connector I2C interface. Driver IC is SSD1306.
Rotary Encoder is from EBAY: https://www.ebay.co.uk/sch/i.html

Count down timer based on PIC16F876A and OLED display module. Included C code and circuit diagram. Time span is 100 minutes. Setting the time is by MINUTES / SECONDS pushbuttons which advances the minutes display by 1 per second. Holding the pushbutton counts faster. Pressing MINUTES and SECONDS together resets the display.
Pressing START / STOP starts the countdown. Pressing START / STOP during count pauses or restarts the counting. When the time reaches zero the buzzer sounds for 30 seconds.
DS18B20 digital sensor has temperature reading of -55 to 99.9 centigrade. The sensor doesn't require calibration. The software updates the reading every 10 seconds.
OLED module is 0.96" 128x64, 4 pins connector I2C interface. Driver IC is SSD1306.

Timer based on PIC16F628A and OLED 0.96", 128x64, SSD1306 driver. Included C code for MPLAB-X, and circuit diagram. Time span is 1 second to 99 minutes. Setting the time is by pressing 4 keys starting with tenth of minutes. * key starts the countdown, # key stops the countdown and zeros the time, to stops the alarm press any key.
Relay drive output is high from start to alarm, it can be used for driving a relay for external functions. OLED brightness can be set in the code.
Buzzer is a piezo crystal.

Timer based on PIC16F628A and OLED 0.96", 128x64, SSD1306 driver. Included C code for MPLAB-X, and circuit diagram. Time span is 1 second to 99 minutes. Setting the time is by Min / Sec pushbuttons. To reset the time press Sec and Min simultaneously. Stop/Start button starts, pauses the countdown and stops the alarm.
Relay drive output is high from start to alarm, it can be used for driving a relay for external functions. OLED brightness can be set in the code.
Temperature sensor is DS18B20, range -55 to 99.9 centigrade.

Timer based on TM1637 4 digits 7 segment module. Time span is 1 second to 99 minutes. Setting the time is by Min / Sec pushbuttons. To reset the time press Sec and Min simultaneously. Stop/Start button starts, pauses the countdown and stops the alarm.
LED brightness can be set in the code, value can be 0-7.
ATtiny is programmed using Arduino IDE and Arduino as ISP. Use these instructions https://42bots.com/tutorials/programming-attiny84-attiny44-with-arduino-uno/ with the difference of burning bootloader with setting "Clock Source 4MHz (external)" and Pin Mapping : "Clockwise".

Timer based on PIC16F628A and TM1637 4 digits 7 segment module. Included C code for MPLAB-X, and circuit diagram. Time span is 1 second to 99 minutes. Setting the time is by Min / Sec pushbuttons. To reset the time press Sec and Min simultaneously. Stop/Start button starts, pauses the countdown and stops the alarm.
Relay drive output is on from start to alarm, it can be used for driving a relay for external functions. LED brightness can be set in the code, value can be 0-7

Timer based on PIC16F628A LCD Module and a watch crystal. Included C code, and circuit diagram. Time span is 1 second to 99 minutes. Setting the time is by Min pushbutton which advances the minutes display by 2 per second, the Sec pushbutton advances the seconds. To reset the time press Sec and Min simultaneously.
The counting generated by the 32768Hz crystal oscillator. The CPU frequency is 4MHz from the internal oscillator.
Egg Timer: CYCLE switch open. Pressing start turns on the LED and relay and starts the countdown. When the time reaches zero the buzzer sounds for 20 seconds. After the alarm the displayed time sets to the recent time.
Repeat Timer: CYCLE switch closed. Pressing start turns on the LED and relay and starts the countdown. At end of count down LED and relay toggle and count down repeats.

Older version of software included. You are free to use the circuit diagram and the software with no limitations.

99 minutes 7 segments LED display, Low cost Arduino Nano with ATtiny88 egg timer. Seconds and minutes are set by buttons. Also buttons for start and stop, pressing together MIN and MINx10 resets the display. Alarm at the end of count down switches on a piezo buzzer. RELAY output is high while countdown, it can be used to drive a relay.
Included ino sketch file for common cathode LED and ino file for common anode LED. When using common cathode LED colon 1K resistor is connected to 5V. When using common anode LED colon 1K resistor is connected to GRD. cc1 is the left digit. The LED I used is 5643BS-1. Brightness is set by the value of the 1K resistors, minimum value is 680R to avoid MC output max current.
ATttiny is programmed using Arduino IDE. Use these instructions https://handsontec.com/dataspecs/module/Arduino/Arduino%20Tiny%2088.pdf

99 minutes 7 segments LED display, Arduino Uno or Nano egg timer. Seconds displayed, minutes are set by buttons. Also buttons for start and stop, pressing together MIN and MINx10 resets the display. Alarm at the end of count down switches on a piezo buzzer.
The code multiplexes the 4 digits at frequency of 120Hz. 7 segment LEDs are common cathode, 4 separate digits can be used with segments a to g linked. cc1 is the digit on the right. The LEDs I used are 5621AS.

Count down timer based on PIC16F628A and a watch crystal. Included C code and circuit diagram. Time span is 100 minutes. Setting the time is by MINUTES / SECONDS pushbuttons which advances the minutes display by 1 per second. Holding the pushbutton counts faster. Pressing MINUTES and SECONDS together resets the display.
Pressing START / STOP starts the countdown. Pressing START / STOP during count pauses or restarts the counting. When the time reaches zero the buzzer sounds for 10 seconds and the display flashes.
Download includes older version that uses low brightness LED.

The oscillator is a watch crystal of 32.768 KHz. The 22pF can vary with different crystal makes.
The 4 digits display is common cathode, the part I used is 5621AS. The 7 resistors of 1K enable current of 25 mA max at figure 8. The seven segments, a to g, of one digit are linked to the corresponding seven segments in the other digits.
The buzzer, piezo buzzer is driven by a frequency of 2KHz.

Timer based on PIC16F628A and a watch crystal. Included ASM, C code, and circuit diagram. Time span is 1 to 99 minutes. Setting the time is by +MINUTE/RESET pushbutton which advances the minutes display by 1 per second, the +MINUTE X 10 pushbutton advances the tens of minutes digit. Pressing START sounds the buzzer for 0.25 sec and starts the countdown. When the time reaches zero the buzzer sounds for 20 seconds and the display flashes 'AL'. Pressing +MINUTE/RESET pushbutton resets the timer and/or stops the buzzer.

Transistors are BC337 or equivalent.
The 2 digits display is common cathode, the 7 resistors of 390 ohms enable current of 6 mA total averages at 3 mA through each segment which is on for half the time. The seven segments, a to g, of one digit are linked to the corresponding seven segments in the other digit.
The 2 digits are multiplexed by pins 17 and 18 that drive the common cathodes.
The buzzer is driven by a frequency of 1KHz, a piezo buzzer for that frequency will give optimal output.