@▷ Darkroom Timer v2.0A for PCB Exposure Box | Diagram for Schematic

Darkroom Timer v2.0A for PCB Exposure Box

This is an improved version of the Darkroom Timer originally created by Stan Ockers (1999). Some extra features were added and the PIC code was modified accordingly

Lamp(s) light at start of timing and turn off when timer reaches zero. A set of leds were added between the minute and second leds which blink during timer operation. 7 segment displays include tails for digits 6 and 9. A bicolor led (common cathode) indicates standby (green) and timer operation (red). Separate PCBs were designed for the led display and the timing system. The power supply is also integrated in the PCB.

The complete schematic, PCB and silkscreen are available in high resolution pdf format. The component values are clearly indicated on the silkscreen. The modified source code is available in asm and hex format (compiled in MPLAB).

Darkroom Timer Schematic

Darkroom Timer PCB

Darkroom Timer Silkscreen

Parts list

System board

R1 - R7 150 Ù 1/4W 5%
R8 - R11 4.7 K
Ù 1/4W 5%
R12 - R13 1 K
Ù 1/4W 5%
R14 100
Ù 1/4W 5%
R15 75
Ù 1/4W 5%
R16 10 K
Ù 1/4W 5%
- C2 22 pF
C3 2200
ìF/25V electrolytic
- C6 100 nF

D5 1N4001/4004/4007
F1 1A Fuse (with socket and cap)
B1 1A Bridge rectifier
IC1 PIC16F84AP Microcontroller (4MHz clock) (with IC socket)
IC2 74HC4543 BCD to 7-segment decoder (with IC socket)
IC3 7812 voltage regulator
IC4 7805 voltage regulator
PL1 14 pin connector
JP2 6 pin header
K1 12V DC / 220V AC Relay
OK1 CNY17-1 Optocoupler

OSC1 4MHz Crystal
Q1 - Q4 BC557/558 PNP Transistror
Q5 BC547/548 NPN Transistor
Q6 BC557/558 PNP Transistror
Q7 2N2222 NPN Transistor
TR1 220VAC/15VAC 1VA PCB Transformer

X1 - X3 2 pin WAGO screw clamp

Led board

D1 - D4 Common cathode 7-segment display
LED1 - LED2 Orange Led 3 mm
JP1 Not a connector, cable is soldered directly on the back of the led PCB

Box front

Bicolor common cathode led (green-red)
3 push-button switches

Switch connections

Connector X1 is connected to the lamp(s)
Connector X2 is connected to 220VAC mains
Connector X3 is connected to the mains switch

Led board is connected to system board via connector PL1

Connector JP2

pin number



START switch


SET switch


SELECT switch


Green bicolor led anode


Red bicolor led anode



Connect the three switches between pins 1, 2, 3 and ground (pin 6). Bicolor led common cathode is connected to ground (pin 6).

Start switch starts the timer. Pressing Start again (before or after timer reaches zero) resets the timer to its initial value.

Set switch sets the time of the current memory position. This is done in a digit‑by‑digit fashion.

Select switch rolls the time values through the 15 memory positions.


The system board

The led board

Connection between system and led boards (PL1)

Connection of switches and bicolor led (JP2) done in breadboard
Normally switches and bicolor led are attached to the front of exposure box

Board testing with externally applied DC on the 7812 regulator (for safety reasons)

A real time video is also available. Pay attention to the blinking leds between minutes and seconds and the bicolor led indicating both operation (red) and standby (green). Unfortunately there is no sound for the relay clicks to be heard !

The second video shows some real action ! A common light bulb turns on and off exactly like in a PCB exposure box.

Download whole project in .doc format (without pdf files) - 3,57MB
Download additional photos in .doc format - 3,88MB
Download whole project in .zip format (with pdf files) - 3,97MB

Photos of the Darkroom timer installed on the exposure unit

The system board, LCD board and switches installed
Notice the common ground for push buttons and bicolor led (JP2 Pin 6)

The darkroom timer during operation

A third video shows the pcb exposure box in operation. A plain lamp was used in this video but when you have UV lamps installed, never leave the top lid open !

Download photoset of the lamps install into the box

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