Wiring up a PIR to a PiFace Digital2

It would help if you could specify the type of PIR you are trying to use? Is it an alarm system type PIR that needs a 12V supply and has a set of NO/NC relay contacts? Or is it a 5V module of sorts that has a logic signal output?

In general, you need to power the PIR (+ve and GND) and get a signal from the PIR. If the PIR outputs a logic level signal, you could use the digital inputs and connect the PIR output to one of them. If the PIR is a relay output, then you will need to configure a pull-up using resistors to provide an appropriate logic signal.

- Gough

It's one of these:-

https://projects.raspberrypi.org/en/projects/physical-computing/13

On mine the pins aren't labelled, but I assume they are standard as in https://howtomechatronics.com/wp-content/uploads/2015/09/PIR-Sensor-Pinout.jpg?x57244

I think you are correct about the pinout:

https://www.mpja.com/download/31227sc.pdf

From my searches, I should connect:-

VCC to GPIO pin 2

OUT to GPIO pin 7

GND to GPIO pin 6

according to https://www.researchgate.net/profile/Zakaria_Rada2/publication/312218161/figure/fig3/AS:449572641218564@1484197878779/GP…

but am unsure how to map these pins to the PiFace Digital2...

Having said that this diagram https://projects-static.raspberrypi.org/projects/physical-computing/0cb2cbd34292a05a668aeea3f291ceb3c7d9cd83/en/images/p…  appears to have the pins inverted....

This diagram http://www.piface.org.uk/static/img/piface_digital_2/pifacedigital2_diagram.png  show two possible locations for VCC and three possible locations for GND. Does it make any difference which I choose? And am I correct in thinking OUT must be one of the eight green terminals?

Operating with and without the PiFace Digital is slightly different. The diagrams you have before are designed for direct GPIO interfacing rather than going through an I/O expander such as the PiFace Digital. The principles are, however, much the same.

The VCC line should be connected to any of the 5V outputs from the PiFace Digital, the GND line should be connected to any of the GND lines on the PiFace Digital and the OUT line can be connected to any one of the input lines on the PiFace Digital. Your code should read the corresponding input line to find out the status of the PIR.

- Gough

I finally managed to get this working connecting the OUT pin to the terminal 0 of the input row.  I'm using the following code:-

# Alarm program by Francesco Balsamo

#

# Using a PIR sensor with inverted logic

#

 

 

from time import sleep

 

 

import pifacedigitalio as pfio

# import piface.pfio as pfio

 

 

pfio.init()

 

 

while(True):

    for pin_number in range(0,4):

        if pfio.digital_read(pin_number) == 0:

            pfio.digital_write(pin_number+4,1) # Using only the leds

        else:

            pfio.digital_write(pin_number+4,0) # Disabling only the leds

This switches on LEDs 7,6,5,4  of which 4 is toggled by the PIR, although I can't figure how the code is supposed to work at all...

I guess this calls for some experimentation. At least I have managed to get the PIR to control one of the LEDs.

The code is pretty simple:

- while (True) runs the code block that follows infinitely.

- for pin_number in range(0,4) basically repeats the code for every pin_number in this set (0, 1, 2, 3)

- if pifo.digital_read(pin_number) == 0 basically says "is this input at pin_number low (as the PIR output goes low when it senses something)?"

- pfio.digital_write(pin_number+4,1) basically says "lets make LED at pin_number+4 (e.g. for pin 0, then LED4 will) turn on"

the other case is for if that isn't the case, it turns off the LED.

This is a rather simple case of polling the input continuously ...

- Gough

The example that was referenced looks like a PiFace Digital rather than a Digital 2 but should be similar.  From my understanding, the first 4 (0-3) inputs/outputs are tied to the switches (S0-S3) so the example skips these to set LEDs 4-7 by offsetting the value from the 'for/range' loop with "pin_number+4" .  However, the "if" statement is only checking for inputs 0-3 which would be one of the switches or an input from 0-3 but then sets an LED, 4 plus pin_number (4-7), which seems odd.  The example also is using 4 PIR sensors, so if you have only 1 PIR sensor then you could remove the for range loop from the code example.

If you want an output on say output 4, then you should put the Out from the PIR sensor to Input 4 on the Digital 2.   However, I found the inputs on the Digital 2 are tied to 5v which causes an issue with the Parallax PIR sensor so I place a 10K Ohn resistor between the input port I am using on the Digital 2 to ground (GND) to ensure the PIR reading is clean.  You can disable the pull-up on the input of the Digital 2 but that causes them to float so you would still have to to either tie it high or low depending on the device connected to the Digital 2.

Reference to PiFace Digital inputs:

https://pifacedigitalio.readthedocs.io/pifacedigital.html#inputs

After some experimentation I came up with:-

import time

import pifacedigitalio as pfio

start_time = time.time()

print('start',time.time())

 

 

pfio.init()

 

 

while(True):

    for pin_number in range(0,1):

        if pfio.digital_read(pin_number) == 0:

            print('hello')

            print('elapsed time',time.time() - start_time)

 

This produces something like:

start 1542805023.9155102

hello

elapsed time 2.0424649715423584

hello

elapsed time 2.0430703163146973

.

.

.

.

hello

elapsed time 4.6204235553741455

hello

elapsed time 4.62097692489624

hello

elapsed time 4.621711015701294

hello

elapsed time 11.211778163909912

hello

elapsed time 11.2123863697052

hello

elapsed time 11.21294355392456

hello

elapsed time 11.213491439819336

hello

elapsed time 11.214038372039795

.

.

hello

elapsed time 13.741790294647217

hello

elapsed time 13.742336750030518

I'm trying to figure out what is going on... I have the Delay Time setting to max, which I think is around 7 secs, so I guess each output of 'hello' is a result of polling which must be taking place every .001 secs over 7 secs.

How would I go about printing a single 'hello' after motion has been detected?

If sensor == 0 set a flag = 0.

print if the flag == 1 and the sensor == 0. 

When the signal == 1 set the flag = 1.

Yeah, what Doug said.

From my experience, a PIR sensor stays active for a period of time after it senses motion, so I would suspect this is what your are seeing.

You could put another while loop after the if statement to have it wait for the input pin change state again such as:

NOTE: This will cause the code to pause while waiting for the pin_state to change to 1 again.

pin_number = 0
pin_state = 1


while(True):


    pin_state = pfio.digital_read(pin_number)
    if pin_state == 0:


        print('hello')


        print('elapsed time',time.time() - start_time)


        while pin_state is 0:
            pin_state = pfio.digital_read(pin_number)
            time.sleep(1)

NOTE: the 'for range' in your code is only looping on 0 so it is a bit unnecessary.

for pin_number in range(0,1):

There are a ton of ways to accomplish this in python so you may have to experiment to find what works best for you.

Did you tie the input pin low with a 10K resistor to ground?  This will ensure the pin is 0 when the signal is not present.

Yeah, what Doug said.

From my experience, a PIR sensor stays active for a period of time after it senses motion, so I would suspect this is what your are seeing.

You could put another while loop after the if statement to have it wait for the input pin change state again such as:

NOTE: This will cause the code to pause while waiting for the pin_state to change to 1 again.

pin_number = 0
pin_state = 1


while(True):


    pin_state = pfio.digital_read(pin_number)
    if pin_state == 0:


        print('hello')


        print('elapsed time',time.time() - start_time)


        while pin_state is 0:
            pin_state = pfio.digital_read(pin_number)
            time.sleep(1)

NOTE: the 'for range' in your code is only looping on 0 so it is a bit unnecessary.

for pin_number in range(0,1):

There are a ton of ways to accomplish this in python so you may have to experiment to find what works best for you.

Did you tie the input pin low with a 10K resistor to ground?  This will ensure the pin is 0 when the signal is not present.

Many thanks for the help. My python programming skills are non existent so would not have managed to come up with this, although I'm wondering if you have a typo in line 18... Also my electronics knowledge is non existent so using resistors is a no no at this time. However I am now able to use my PIR connected to my PiFace Digital2 to trigger various actions. The next step is to get the PIR to do something with the relays.

Have a look at the post i made yesterday.  The code there does work in my config.

For the relays, they should be tied to output pins 0 and 1 and would be controlled as such:

pifacedigital.output_pins[0].turn_on()

The Relay section of the following doc for the PiFace Digial as the commands to control the relays.

https://media.readthedocs.org/pdf/pifacedigitalio/latest/pifacedigitalio.pdf

Note: Udemy has a number of courses discounted to $9.99 for Black Friday in the US.  That might be a good place to find a Python course that will assist your learning.