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.
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
R1 – R7 150
R8 – R11 4.7 KÙ
R12 – R13 1 KÙ
R14 100 Ù
R15 75 Ù
R16 10 KÙ
C2 22 pF
C3 2200 ìF/25V
C6 100 nF
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
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
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
X1 – X3 2 pin
WAGO screw clamp
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
Bicolor common cathode led
3 push-button switches
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
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
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
Board testing with externally applied DC on the 7812
regulator (for safety reasons)
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
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