Darkroom Timer v2.0A for PCB Exposure Box

PIC based automatic timer for a 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 Ω  1/4W 5%

R8 – R11           4.7 KΩ  1/4W 5%
R12 – R13         1 KΩ 1/4W 5%
R14                   100 Ω 1/4W 5%
R15                   75  Ω 1/4W 5%
R16                   10 KΩ 1/4W 5%
C1 – C2             22 pF
C3                     2200 μF/25V electrolytic
C4 – 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

JP2 pin number - Connection
1 - START Switch
2 - SET Switch
3 - SELECT Switch
4 - Green bicolor led anode
5 - Red bicolor led anode
6 - Ground

Operation

• 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.

Photos

The system board

The led board

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)

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

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