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by Paul Moir
Fri Apr 25, 2014 6:51 am
Forum: Automation, sensing and robotics
Topic: Safe shutdown - Simple way
Replies: 31
Views: 5235

Re: Safe shutdown - Simple way

If you look closely at a supercap's specs, they are not exactly durable. Esp. at elevated temperatures.
No free lunches I'm afraid...
by Paul Moir
Sat Apr 19, 2014 6:30 am
Forum: Automation, sensing and robotics
Topic: plc automation
Replies: 23
Views: 10933

Re: plc automation

Which PLC does matter a lot. Libmodbus works pretty easily with PLCs that support MODBUS. If MODBUS is an option that's the way to go. Most PLCs have some proprietary or might-as-well-be-proprietary communication scheme, but many support the ancient MODBUS protocol. This is the real backwater of com...
by Paul Moir
Sat Mar 15, 2014 8:00 am
Forum: Automation, sensing and robotics
Topic: "Pushing" Buttons with GPIO
Replies: 7
Views: 4728

Re: "Pushing" Buttons with GPIO

Yeah, exactly. The hard way is to reverse engineer the buttons on the FOB and then figure out how it's chip interfaces with them. The goal will be using a transistor or FET in your solution as the switching element. 'Course your FOB will need a common ground with the Pi, and you'll have to know a bi...
by Paul Moir
Thu Mar 13, 2014 6:24 am
Forum: Automation, sensing and robotics
Topic: "Pushing" Buttons with GPIO
Replies: 7
Views: 4728

Re: "Pushing" Buttons with GPIO

Sure. The dumbest/easiest way would be to use a reed relay. Digikey has one that runs at 3v/6ma which would be directly compatible with the GPIO if you add a freewheeling diode. About $5 each.
by Paul Moir
Fri Mar 07, 2014 5:04 am
Forum: Automation, sensing and robotics
Topic: Raspberry pi party light show controller
Replies: 10
Views: 22360

Re: Raspberry pi party light show controller

Holy Cramolie, is that how you do a colour organ in 2014? FFT on a 700MHz processor probably running PWM to a bunch of amplifiers to switch the lights, and no doubt some 16 bit 44.1KHz A /D for some retro gear? I've got a fist full of TRIACs and capacitors that would like to have a word with you! :)...
by Paul Moir
Tue Mar 04, 2014 10:32 pm
Forum: Beginners
Topic: Using wiringPiISR
Replies: 8
Views: 10811

Re: Using wiringPiISR

Ints are binary and your ISR will have to deduce if it was the rising edge or the falling edge that caused the interrupt. So the first thing you would want to do in your ISR code is to read the GPIO pin and determine if it's high or low and therefore whether it was a low-high or high-low transition ...
by Paul Moir
Mon Feb 24, 2014 6:22 pm
Forum: Beginners
Topic: 4066 CMOS switch
Replies: 4
Views: 1626

Re: 4066 CMOS switch

Ok, no problem then with a 3.3v power supply. At least I think it should work perfectly normally down that low. The "On" state resistance gets higher as Vdd drops, so I guess you can expect a few K across each switch.
by Paul Moir
Mon Feb 24, 2014 7:55 am
Forum: Automation, sensing and robotics
Topic: Linear Actuator for Door Lock Demo
Replies: 30
Views: 13062

Re: Linear Actuator for Door Lock Demo

Best bet is to optocouple it. That way no matter how bad you screw up the 12v side you're pi will never see it.
by Paul Moir
Mon Feb 24, 2014 7:34 am
Forum: Beginners
Topic: 4066 CMOS switch
Replies: 4
Views: 1626

Re: 4066 CMOS switch

You can directly connect. No need for a resistor. That said, what are you supplying your 4066 at? At 5v you need 3.5v minimum on a control line to switch it on, so you would have to level shift in that case. Check the datasheet and you'll see what I mean. EDIT: You know, the 74HCT4066 (but not the 7...
by Paul Moir
Thu Feb 20, 2014 8:15 pm
Forum: Automation, sensing and robotics
Topic: Linear Actuator for Door Lock Demo
Replies: 30
Views: 13062

Re: Linear Actuator for Door Lock Demo

Possibly the easiest way would be to just get a cheap servo motor from a hobby store or ripped out of a cheap RC toy car and attach it with an arm (ie, a bent bit of clothes hanger) to a normal manual door bolt. You really don't need a linear actuator since you only need two positions. As for attach...
by Paul Moir
Tue Feb 11, 2014 1:05 am
Forum: Automation, sensing and robotics
Topic: Weight measuring (5kg) with Raspberry Pi, MCP3008
Replies: 5
Views: 2867

Re: Weight measuring (5kg) with Raspberry Pi, MCP3008

That's more like it. That is an A/D converter and not a scale as such, therefore you'll have to code the scale functions yourself. Note that with the gain at 128, it's +-20mV full scale with a 5v supply. Since you're only getting 1mV/V you only get 0-5mV out of it with a 5V supply. So the maximum ra...
by Paul Moir
Sat Feb 08, 2014 10:31 am
Forum: Automation, sensing and robotics
Topic: Weight measuring (5kg) with Raspberry Pi, MCP3008
Replies: 5
Views: 2867

Re: Weight measuring (5kg) with Raspberry Pi, MCP3008

No, that won't work. The excitation voltage is 5-10v but the output from a load cell is typically very low. Like a few millivolts low. Note below it has a rating of around 1mv/v; that means that with 10v of excitation and with 5kg sitting on it, you get a whopping 10mv out. And I presume you want to...
by Paul Moir
Wed Feb 05, 2014 3:33 am
Forum: Interfacing (DSI, CSI, I2C, etc.)
Topic: pi-face no NO on Relais 1 ?
Replies: 2
Views: 630

Re: pi-face no NO on Relais 1 ?

Sounds like a fault to me. You are testing between NO and common, not NC right? (Dumb question but best to make sure)

Bad connector, board or relay. Not many options other than that. 0 should function just like 1.
by Paul Moir
Sun Jan 26, 2014 8:11 am
Forum: Raspbian
Topic: Resilience to hard shutdown
Replies: 5
Views: 1458

Re: Resilience to hard shutdown

That's what I do too: it makes a lot of sense for an embedded system. I use IPE R1 which is already setup readonly and a usb stick for logged (R/W) data. The stick is automounted with usbmount, and it's mounted with the option "sync" to force all writes immediately with no caching. An even better me...
by Paul Moir
Tue Jan 21, 2014 6:05 am
Forum: Interfacing (DSI, CSI, I2C, etc.)
Topic: GPIO Electronics Advice - Voltage Spike Protection
Replies: 13
Views: 2494

Re: GPIO Electronics Advice - Voltage Spike Protection

The RFI you're producing can be effectively suppressed by using a modest resistance (~2k) in your high voltage line. You can use resistance or inductive type spark plugs, or normal automotive ignition wire which has a high resistance. That stuff is made with a carbon conductor rather than a metal on...
by Paul Moir
Sun Jan 19, 2014 1:49 am
Forum: General discussion
Topic: Safe to use 1/4" 4-40 metal standoffs?
Replies: 6
Views: 3390

Re: Safe to use 1/4" 4-40 metal standoffs?

Ok, good, I was relying on memory there. I couldn't remember if I had tried 6-32 or 4-40. Or maybe it was 3mm. But a normal 4-40 nut is 3/16" wrench size so about .216 at the corners, so that might be fine anyway, and if not a fiber washer would be a good idea. Speaking of which, can you get a 3/16"...
by Paul Moir
Sun Jan 19, 2014 1:01 am
Forum: General discussion
Topic: Safe to use 1/4" 4-40 metal standoffs?
Replies: 6
Views: 3390

Re: Safe to use 1/4" 4-40 metal standoffs?

I think they'll be too big. There isn't much clearance (sorry, all my rev b boards are in things right now so I can't measure). I used 5mm standoffs and they're fine.
But why so big? I don't think 4-40s will fit the 2.5mm mounting holes anyway.
by Paul Moir
Fri Jan 17, 2014 4:45 am
Forum: Interfacing (DSI, CSI, I2C, etc.)
Topic: GPIO detects very small currents
Replies: 3
Views: 832

Re: GPIO detects very small currents

Another is to suppress the meaning of the interrupt with logic that says a key must be held for n milliseconds before it is counted as pressed. You can debounce your switches this way too. I have a pi setup with a not-very-noisy solenoid attached which would generate spurious interrupts on the GPIO....
by Paul Moir
Fri Jan 10, 2014 10:26 pm
Forum: Interfacing (DSI, CSI, I2C, etc.)
Topic: GPIO Electronics Advice - Voltage Spike Protection
Replies: 13
Views: 2494

Re: GPIO Electronics Advice - Voltage Spike Protection

Personally I think optoisolation like you're suggesting is best. It can be wickedly hard to properly suppress spikes like that from ignition circuits since they get induced directly into wires (capacitive or inductive), and you really have to consider the possibility of a coil insulation failure . Y...
by Paul Moir
Fri Jan 10, 2014 10:01 pm
Forum: C/C++
Topic: GPIO matrix keyboard driver
Replies: 3
Views: 2015

Re: GPIO matrix keyboard driver

You can use blocking diodes to discriminate during multiple keypresses: http://www.headfuzz.co.uk/midihack2 This also solves the output short problem, so you just use weak pull-downs on the input GPIOs as long as you only turn on one output GPIO at a time. The cost is more parts (diodes) on larger m...
by Paul Moir
Fri Jan 10, 2014 9:40 pm
Forum: HATs and other add-ons
Topic: cardreader on com port?
Replies: 8
Views: 3123

Re: cardreader on com port?

If the card reader needs hardware handshaking, you can just fake it by wiring DTR to DSR and RTS to CTS. If the card reader does not generate DSR or CTS (assuming it's a DCE, pretty safe since it works with your computer) and only listens to DTR and/or RTS, you'll have to wire DTR and RTS to somethi...
by Paul Moir
Fri Jan 10, 2014 5:22 am
Forum: Automation, sensing and robotics
Topic: PiFace: High Voltage/Amperage Relay Board?
Replies: 8
Views: 3965

Re: PiFace: High Voltage/Amperage Relay Board?

If you have enough GPIOs, you might consider skipping the piface entirely. You can directly drive those SSRs with the GPIO pins. I did a project using a piFace about a month ago. I'm still not entirely sure it was worth using. It did save some time, but the implementation isn't fantastic in either d...
by Paul Moir
Thu Jan 09, 2014 5:16 am
Forum: HATs and other add-ons
Topic: PiFace Output
Replies: 2
Views: 1347

Re: PiFace Output

A quick question: what kind of loads are you turning on and off? They don't happen to be largish (say > .25kW) motors or big transformers or anything with a large electromagnet by chance? If so there are some considerations with relays. Light bulbs, heaters, etc are no problem.
by Paul Moir
Thu Jan 09, 2014 5:01 am
Forum: HATs and other add-ons
Topic: PiFace Output
Replies: 2
Views: 1347

Re: PiFace Output

If you want to use relays with 12v coils, you'll have to remove JP4 and install snubber diodes on your relays. The two relays on the piface already have some so you don't need to fix them. JP4 connects in some built in snubber diodes, but they're referenced to +5v so you have to disable them. No pro...
by Paul Moir
Thu Jan 09, 2014 4:32 am
Forum: Automation, sensing and robotics
Topic: PiFace: High Voltage/Amperage Relay Board?
Replies: 8
Views: 3965

Re: PiFace: High Voltage/Amperage Relay Board?

Look nice and compatible. Pretty much guaranteed on after 6ma makes them directly compatible with the Pi's GPIO using only a current limiting transistor. Only real issue worth considering with SSRs is that they almost always fail on. That can be a problem in some applications, but it hasn't stopped ...

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