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Awesome Interface
Raspberry Pi GPIO Interface
Board
Features 8 x 0‐24V inputs.
8 x 0‐24V outputs that can supply up to 1.2A total.
8 x Pi GPIO LED status indicators.
8 x Input switches to configure the inputs and outputs.
Overview
The raspberry pi GPIO interface board is designed to isolate 8
digital inputs and outputs from the Pi. The board can act as
general purpose isolation buffers with up to 1.2A total output
current and up to 24V inputs and outputs.
Inputs Each Input utilizes an open collector comparator followed by
an optocoupler with a pull up resistor on the collector of the
output transistor. The optocoupler can be bypassed for faster
switching times, and the pullup voltage can be selected Figure 1: Input Circuit
8x 0‐24V inputs (3V‐24V is logic high). Input supply voltage: The board can be configured to use the
Pi’s supply with an onboard jumper or an external 7‐15V
supply.
5V supply (from Pi) will draw 98mA when all inputs are off (‐7mA per ON input).
3.31200
This is the current sinking from the input.
Outputs The outputs are simple L272M IC’s with an output voltage
dependant on the supply voltage you apply to the PCB.
Figure 2: Output Circuit
Output Features 4‐24V supply will draw 50mA + output current.
Output voltage = Supply voltage ‐ 0.55V. Max output voltage set by supply up to 23V
Output current max = 1.2A total, and 1A per output (source or sink). Table 1: 26 Pin Header Pin assignment (Corresponds to Raspberry Pi)
Pin Function R‐Pi Name
1 3.3V Pullup
supply
3.3V
2 5V input IC
supply
5V
6 GND 0V
7 I/O 1 GPIO 4
11 I/O 2 GPIO 17
13 I/O 3 GPIO 27
15 I/O 4 GPIO 22
12 I/O 5 GPIO 18
16 I/O 6 GPIO 23
18 I/O 7 GPIO 24
22 I/O 8 GPIO 25
Awesome Interface
Awesome Interface
Configuration Options
Power Options:
Figure 3: Input supply selection jumper (configured for external supply)
Jumper configuration: Pins 1 &2 = 5V from raspberry pi
Pins 2 &3 = 5V from regulator (requires >7V on the input voltage terminal).
Pull up supply voltage If the interface is used without a raspberry pi a pull up voltage for the inputs should be applied to pin1 of the 26
pin header.
5V can be routed from the regulator to this pin by bridging all 3 pins of the piSupply header, and adding a jumper
between pin 1 and 2 of P1 or P2.
NOTE: pins 1 and 2 of P1 and P2 must not be shorted when a raspberry pi is connected or you will damage the 3.3V
supply.
Figure 4: Jumper locations
The 26 Pin headers are in parallel to allow access to the other GPIO pins while the buffer is in use.
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Switching Speed: For high speed inputs the optocouplers can be omitted by bridging pins 2‐7 and 3‐6 of the optocouple IC sockets as
shown with red lines in figure 4. The optocouplers can be selectively removed, so you can have only 2 or more
opto‐isolated inputs while the rest are higher speed isolated only by the lm339. Note each opto‐coupler supports 2
inputs.
If no optocouplers will be used resistor network RN2 can be omitted as well.
Figure 5: PCB with optocouple bridges Indicated in Red
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Table 2: Input Response Times
WITH OPTOCOUPLERS
Parameter Value Unit RL 1k2 ohms
Rf 620 ohms
Ton(1.8V) 4 µs
Ton(3.3V) 12 µs
Toff(0.8V) 7 µs
Toff(0.3V) 10 µs
3.3V supply drain 4.85 mA / LOW input
WITHOUT OPTOCOUPLERS
Parameter Value Unit RL 1k2 ohms
Ton(1.8V) 1.2 µs
Ton(3.3V) 1.4 µs
Toff(0V) 0.2 µs
Toff(0.3V) 0.1 µs
3.3V supply drain 2.65 mA / LOW input
For output response times refer to the L272M datasheet from ST.
Input/Output indicators The LED bar graph indicates the status of the bit at the raspberry pi pin. The leftmost bit is lit for input supply
voltage present, second from the left is for output supply voltage presence.
The indicators can be entirely omitted along with the 74LS540N and RN3 with no modifications necessary.
Switches The switches allow you to disconnect the input from the raspberry pi pin. They should be opened when the
corresponding output is in use, to avoid conflicts between an output pin on the Pi and an input voltage.
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Figure 6: Complete PCB
Input Switches
26 Pin header matching Raspberry Pi
Input supply voltage selection
Input Supply
Optocouplers
LED driver IC (The LED bar indicates the state of the corresponding pin on the 26P header & the
presence external output or input supply voltages).
Outputs Inputs Output Supply
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Figure 7: Schematic
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Figure 8: PCB
The PCB can be flipped over and mounted underneath the raspberry Pi. If this is the intended use the PCB
terminals can be mounted on the underside of the board. Two mounting holes are included with the same
spacing as the Pi (when the PCB is upside down)