LPT relay board circuits

[mortenbs.com - Website of Morten B. Sørensen]

LPT relay board circuits | mortenbs.com
Last updated 2010.

Before we had the Arduino to control electronics from a PC through USB, I used to build some LPT-based circuits.
Today I just use the Arduino for my robotic projects and small circuits.
Still it was fun to explore the LPT port, and working with LPT in Delphi...

LPT Relay board (2006)

Bottom view

LPT relay board circuit with 10 car relays
10 Solid state relays from a car being tested by LPT board circuit in different patterns and speed.

[LPT relay board circuit with 10 car relays]

The LPT port (DB25 LPT Connector)
The LPT port runs up to 5 volt DC. There is 3 bytes (each of 8 bits) to control the port.
[DB-25 LPT Connector]

Pin	Byte	Bit	LPT Port	LPT board	Behaviour
01	CONTROL	0	Strobe	Output 09	5v when off (inv)
02	DATA	0	Data Pin 1	Output 01	5v when on
03	DATA	1	Data Pin 2	Output 02	5v when on
04	DATA	2	Data Pin 3	Output 03	5v when on
05	DATA	3	Data Pin 4	Output 04	5v when on
06	DATA	4	Data Pin 5	Output 05	5v when on
07	DATA	5	Data Pin 6	Output 06	5v when on
08	DATA	6	Data Pin 7	Output 07	5v when on
09	DATA	7	Data Pin 8	Output 08	5v when on
10	STATUS	6	Acknowledge	Input 01	off when ground (inv)
11	STATUS	7	Busy	Input 05	on when ground
12	STATUS	5	Out of paper	Input 03	off when ground (inv)
13	STATUS	4	Select status	Input 04	off when ground (inv)
14	CONTROL	1	AutoFeed	Output 10	5v when off (inv)
15	STATUS	3	Error	Input 05	off when ground (inv)
16	CONTROL	2	Initialize	Output 11	5v when on
17	CONTROL	3	Select	Output 12	5v when off (inv)
18	-	-	-	-	-
19-25	-	-	Ground	Ground	-

Single LPT output-pin
For each output a transistor gate was used (to invert 5 volt of the pin to ground for external supply).
[Gate_5v-12v]

An output pin is connected to a BC547 NPN transistor, through a 1N4448 diode (75V/0,1A) (to protect the port).
The BC547 transistor requires only <1 volt (through a ~3,3k resistor), to open for ground on the collector pin.
Also a ~10k resistor was used for transistor pull-down.
"Optional gate 2" uses a ~680 ohm resistor for the BD136 PNP transistor (Converting ground to external + 12 volt DC.)

The LPT board
First a >1000uF capacitor and a diode, to give the board an internal power supply.
Then a transistor gate for each 12 outputs, and further 5 transistors for input feedback.
All outputs connected to LEDs, and also to pins on the IDE connector.

In this project the "optional gate 2" for each output, was placed on seperate boards as extensions.

[BD135] Expansion board