Home Projects Oscillators and timers Darkroom Timer v2.0A for PCB Exposure Box



Darkroom Timer v2.0A for PCB Exposure Box
PIC based automatic timer for a PCB exposure box

author: Vassilis Papanikolaou - vpapanikgmail.com

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   1/4W 5%
R8 - R11             4.7 K
  1/4W 5%
R12 - R13           1 K
  1/4W 5%
R14                   100
  1/4W 5%
R15                   75  
  1/4W 5%
R16                   10 K
  1/4W 5%
- C2              22 pF
C3                     2200
F/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


120x90 spot



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