Lo-tech GPIO Interface Board

The Lo-tech GPIO Interface Board enables the connection of the RaspberryPi Model B+ to the outside world, with 4 inputs and 8 outputs. The board has been designed to fit directly on top of the RaspberryPi Model B+ and features low power consumption, being powered directly from the RaspberryPi.

Specifications

• Small form-factor:
  • Mounts directly to RaspberryPi device and fits within device footprint (65x56mm)
  • 2.8mm mounting holes aligned with RaspberryPi device mounting holes
  • 11mm spacing between boards (use 11mm standoff)
• Powered entirely from GPIO header - no separate power connection required
• Inputs:
  • Four independent opto-isolated inputs with screw terminals
  • Recommended input levels: 3.3V to 15V
• Outputs:
  • Four 5V TTL outputs with screw terminals
  • Four on-board LEDs (2-pin headers can be mounted in place of LEDs, if required)
• Single control GPIO control to enable/disable all outputs
• Total assembled weight: 34g (about 85g including RaspberryPi B+ and screws)

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Note: this board does not provide the auto-configuration capabilities defined in the RaspberryPi HAT specification; GPIOs must be manually configured during program initialisation. Pull-ups are provided where necessary to prevent spurious output values.

Inputs

Note: GPIO numbering follows the BCM numbering

The board provides four inputs (GPIOs 22, 23, 24, and 27), each interfaced through an H11L1M opto-isolator. The inputs require 2mA drive, which should be set via appropriate value of RN1. It is therefore possible to use inputs from almost any low-voltage source (note: never connect mains voltage to this board).

Input Logic Level	RN1 Value
15V	6K8
12V	4K7
9V	3K3
6V	2K2
5V	1K5
3.3V	1K

RN1 is Bourns 4608X-102 type (isolated, 8-pin 4-element)

Outputs

Note: GPIO numbering follows the BCM numbering

The board outputs are driven by a 74ACT245N Octal Bus Transceiver. The transceiver gate is controlled by GPIO 21; initialisation process for outputs is:

1. Set GPIOs 5, 6, 12, 13, 16, 19. 20, 21, 26 to output mode
2. Output value 0 (or 1, as required) to the output channels 5, 6, 12, 13, 16, 19, 20, 26
3. Output value 0 to GPIO 21 to enable the bus transceiver

The eight 5V TTL outputs divided into:

• four outputs via screw terminals, for connection to external devices - GPIOs 5, 6, 12, 13
• four outputs to on-board LEDs - GPIOs 16, 19, 20, 26

The total draw across all outputs should not exceed 180mA, and note that power for the outputs is sourced from the RaspberryPi 5V rail.

• Choose RN3 value to match required output impedance (i.e. 100-Ohm)
• Choose RN4 value to provide appropriate current through attached LEDs
• Ensure total current draw will not exceed 180mA
• RN3 and RN4 are Bourns 4608X-102 type (isolated, 8-pin 4-element)

Bill of Materials

Part	Value	Device	Package	Qty	Farnell	Mouser
C1..C5	0.1uF (100nF)	Ceramic Capacitor	THT	4	2112751	594-K104K15X7RF5UH5
C6, C7	10uF	Electrolytic Capacitor	E2-5	2	8767084	647-UVR1C100MDD
HD1, HD2, HD5 (Farnell parts)	1x2 Screw Terminal	MULTICOMP MC000045	THT	3	2008016	-
HD3, HD4 (Farnell parts)	1x2 Screw Terminal	MULTICOMP MC000044	THT	2	2008015	-
HD1, HD2, HD3 (Mouser parts)	8-way screw terminal (3.81mm)	FCI 20020327-D081B01LF	THT	1	-	649-220327-D081B01LF
HD4, HD5 (Mouser parts)	5-way screw terminal (3.81mm)	20020327-D051B01LF	THT	1	-	649-220327-D051B01LF
IC1..IC4	-	H11L1M Optocoupler, Schmitt Trigger	DIP-6	4	1021127	512-H11L1M
IC5	-	74ACT245N Octal Bus Transceiver	DIP-20	1	1739622	595-SN74ACT245N
LED1..LED5	-	3mm LED	THT	5	1581112	645-521-9432F (red) 859-LTL-2231AT (green)
P1	-	SOCKET, PCB, 0.1", 20+20WAY	THT	1	7992033	855-M20-7832046
R1	270R	125mW Carbon Film Resistor	THT	1	2329504	299-270-RC
R2	10k	125mW Carbon Film Resistor	THT	1	9342419	299-10K-RC
RN1, RN3, RN4	See table*	SIL-8 Resistor Network (Bournes 4608X-102 type)	THT	3	See table*	See table*
RN2	10k	Bournes 4605X-101-103LF	THT	1	9356061	652-4605X-1LF-10K
DIL Socket, 8-pin	-	SOCKET, VERTICAL, 1ROW, 8WAY	THT	1*	3419101	571-215297-8
IC Socket (DIP20)	-	-	DIL20	1	1101349	571-1-2199298-6
IC Socket (DIP6)	-	-	DIL6	4	1077343	571-1-2199298-1

*For values of RN1, RN3 and RN4, see tables in Inputs and Outputs sections. Three components are needed for each board, with values chosen to match the desired input voltage levels and output drive current. These components can be changed after construction if SIL-8 sockets are mounted to the board (one socket is specified in the above component list for RN1, since this value is most likely to be changed to interface to different input voltage levels). Component options are:

Value	Part	Farnell	Mouser
100R	4608X-102-101LF	9356509	652-4608X-2LF-100
270R	4608X-102-271LF	9356622	652-4608X-2LF-270
1k	4608X-102-102LF	9356517	652-4608X-2LF-1K
1k5	4608X-102-152LF	9356568	652-4608X-2LF-1.5K
2k2	4608X-102-222LF	9356592	652-4608X-2LF-2.2K
3k3	4608X-102-332LF	9356673	652-4608X-2LF-3.3K
4k7	4608X-102-472LF	9356703	652-4608X-2LF-4.7K
6k8	4608X-102-682LF	2321777	652-4608X-2LF-6.8K

Schematic

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Use of this design is provided subject to the lo-tech.co.uk Terms and Conditions. Copyright (c) 2014 Peacon Ltd.

Programming

Any library suitable for use with the Raspberry Pi Model B+ can be used, for example WiringPi. The GPIO channels used are:

BCM GPIO	Purpose	Notes
5	Output (5V TTL)	Screw terminal output
6	Output (5V TTL)	Screw terminal output
12	Output (5V TTL)	Screw terminal output
13	Output (5V TTL)	Screw terminal output
16	Output	Onboard LED
19	Output	Onboard LED
20	Output	Onboard LED
21	Output Control	Output 0 to enable all other outputs; Output 1 to set high-Z state
22	Input	See note
23	Input	See note
24	Input	See note
26	Output	Onboard LED
27	Input	See note

Note: The specified H11L1M opto-isolator has an LED forward voltage of 1.2V and requires 2mA drive.

See Also

• RaspberryPi Boards
• Lo-tech PCB Shop