A stupidly simple way to connect 3.3V logic to a Sainsmart relay board

Rodney

Thanks for your images and investigations.

I'm puzzled how a 10M probe will cause the led to partially illuminate, and coupled with the voltages you are seeing, would suggest there is something drawing current, hence the voltages you observe.

I note the Sainsmart site now has some links to documents.

This is from the pdf (in the zip file) and gives the relay specs http://www.songle.com/pdf/2008961512231004.pdf

Unfortunately errors have crept in, as the relay manufacturer spec is 89mA at 5v, and the sheet claims 400mA when all relays ON  ...someones maths is wrong.

Unfortunately they are missing the excellent documentation provided by Terry King at Yourduino.com

http://yourduino.com/sunshop2/index.php?l=product_detail&p=218

This shows that there is a 1k resistor in series, which if reduced to 220 ohms will ensure reliable triggering on 3v3.

So you have two options.

1. Is to add a 270 ohm resistor across the 1k (gives 212 ohm)

2. Short out the indicator led and leave the 1k.

either way involves some careful soldering.

I have a 4 way version, so if I get a chance I'll do some checks.

In the meantime I am impressed that they have removed the board around the common, and thereby provided a physical seperation between what could be 230vAC and the rest of the low voltage tracks.

mark

Rodney

Thanks for your images and investigations.

I'm puzzled how a 10M probe will cause the led to partially illuminate, and coupled with the voltages you are seeing, would suggest there is something drawing current, hence the voltages you observe.

I note the Sainsmart site now has some links to documents.

This is from the pdf (in the zip file) and gives the relay specs http://www.songle.com/pdf/2008961512231004.pdf

Unfortunately errors have crept in, as the relay manufacturer spec is 89mA at 5v, and the sheet claims 400mA when all relays ON  ...someones maths is wrong.

Unfortunately they are missing the excellent documentation provided by Terry King at Yourduino.com

http://yourduino.com/sunshop2/index.php?l=product_detail&p=218

This shows that there is a 1k resistor in series, which if reduced to 220 ohms will ensure reliable triggering on 3v3.

So you have two options.

1. Is to add a 270 ohm resistor across the 1k (gives 212 ohm)

2. Short out the indicator led and leave the 1k.

either way involves some careful soldering.

I have a 4 way version, so if I get a chance I'll do some checks.

In the meantime I am impressed that they have removed the board around the common, and thereby provided a physical seperation between what could be 230vAC and the rest of the low voltage tracks.

mark

"Unfortunately errors have crept in, as the relay manufacturer spec is 89mA at 5v, and the sheet claims 400mA when all relays ON  ...someones maths is wrong."

The 400ma is actually correct.  I have measured the coil current and it is typically around 50 - 60ma with a 5V supply (400 - 480 ma with all 8 coils activated).  Perhaps you need to take in account all the circuitry associated with the coil and how fully the two transistors are turned on.

Now the 15 - 20ma input current spec definitely is wrong.  Each input requires 1.5 ma (without any circuit modification).  That spec is either a typo, or when they state "each" they mean each board, not each relay.

"This shows that there is a 1k resistor in series, which if reduced to 220 ohms will ensure reliable triggering on 3v3."

Actually it states something like a 220 ohm resistor, so not sure it was even tested or checked whether there would be too much current flow, so since you are going to try this, and since there are no spec sheets for each part on the relay board, I would slowly lower the resistance of the resistor until you get a reliable activation, keeping in mind max current for a typical opto LED and surface mount LED.

and since there are no spec sheets for each part on the relay board

I did run into that problem.

Using Terrys figures with 2mA current, a 150 Ohm resistor is required, so a 220 will drop the current to about 1.3mA.

I would be happy using a 3v3 voltage and a 150 ohm resistor which should give 2mA sink current.

Thanks for clarifying the relay draw. As you say the sum of the components is enough to make the difference.

mark

Those current values make sense and are in the ballpark of what is needed to get it to work.  When I first got the board a year ago I hooked the Vcc to 3.3V and used a LOW signal to activate it.  I was only measuring 0.49ma current flow through the opto circuitry and the relays were on the hairy edge of working (Sometimes they would work, sometimes they would not).  Switching to using a transistor on the input, 5V for Vcc, and using an active HIGH signal, the current increased to a measured 1.48 ma and the relays worked every time and with a much louder solid click.  In fact I have a separate opto that I bought to operate a 30Amp rated relay that I use to control AC voltages which uses a 1k resistor and is activated directly from the GPIO pin without any additional circuitry and it uses exactly 2.0 ma measured (I selected a Toshiba TLP624-4(F) because the spec sheet calls for a very low recommended operating condition of 1.6 - 2.0 ma).

I had thought of modifying the relay card originally (change the resistor, completely short the external LED, etc.), but I was building a fireworks launcher, and for obvious reasons I didn't want an active LOW circuit.  I just completed that project in the time for the Fourth of July and everything worked out great.  It is a wireless battery operated headless portable fireworks launcher which I operate from my smartphone.  I can launch from a distance of up to 400 feet.  At launch time it is selectable via software for either sequential or simultaneous launches, launch port selection, fuse burn time, and delay time between launches.

and for obvious reasons I didn't want an active LOW circuit

I was wondering what you had against it when I saw the extra board, and components.

But for your purpose it makes sense.

In your application a short on the control wire will NOT launch thereby making it a 'fail to safe' system.

Thanks

Mark

There is more to it than that.  You have to take in consideration the state of the GPIO pins at first boot.  Even though most of the GPIO pins are set to INPUTS and LOW, they are floating, so I have added a 10k resistor from each GPIO pin to ground to insure a LOW at boot, before I have a chance to set the pins to OUTPUTS and LOW in software, which is done by a script later in the boot process.  As an added precaution, I also have a separate battery with a separate power switch which activates the high current launch line of my circuitry (relay outputs), which I only toggle after the boot process is complete.  I also have various error checking in my C code, including the capture of a hypothetical emergency CTRL-C exit out of my code and setting of all the pins to OUTPUTS and LOW before the program closes.

Very well thought out and designed.

I wouldn't mind getting the info on which pins are low and high at boot.

Perhaps you could PM me some time.

Cheers

Mark

Originally, because things were acting funny on the GPIO pins when I first tried them....some LED's turning on at boot, others not, I thought I may have a defective pi (This was my first time working with GPIO's). Then after measuring the pins and doing a bit of reading on the web, I just picked the pins that were low at boot. Since then, Gordon over at the raspberry pi forums made a couple of nice charts for the REV1 and REV2 boards. The only difference I saw on his REV 1 board chart was that I was seeing a HIGH on GPIO8 at very first boot.

Here are the charts:

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=44&t=35321

As noted, the GPIO pin condition at boot can change after a kernel upgrade, and I imagine a firmware update as well (Thus my layered approach to safety related features in my launcher). However, that said, I have never come across this problem in a years time, so it is very rare.

" I was wondering what you had against it when I saw the extra board, and components."

Actually I never posted a picture of my hookup, but yes, I imagine it is similar to the ones posted.  On the raspi foundation web site front page, they have a post mentioning that they are accepting posts of projects this week from users because Liz is going on vacation, so I requested if they would be interested in my project, but I never heard back, and the week is almost over.  I am not going to go through the trouble of taking pictures, making videos, a long writeup, etc., only to have them throw it in the trash bin.  Thier loss.  If hosting your own web site was a bit cheaper, I would just get my own domain and post everything.....I am still mulling that over.  I kinda better find a job first, and maybe pay off all these extraneous pi electronic parts that turned my pi into an order of magnitude more expensive purchase !

Sorry I jumped from a picture and description into an assumption.

Actually I never posted a picture of my hookup

I can't blame you at all.

I'll have a word to the publisher of the magazine I write the occasional bit for and see if he's interested in a RPi article from overseas, that has some good engineering design.

If hosting your own web site was a bit cheaper

I found a few hosts that were free and with very high storage space available.

We are lucky that the domain price is quite low ($23 per year).

Mark

The cheaper domain registrations here charge only $11 a year.  The popular and reliable web hosting companies typically charge $8 a month (1 year subscription) or $6 a month (3 year subscription) with the domain charge included, for the regular not too crippled hosting plan.  Hostgator and Bluehost seem very popular.  However, they both play games with their prices and features, like when they advertise on the front page $3.95 a month and then you find out you have to sign up for 3 years (sometimes for a crappy plan), or they include web tools that require no HTML coding (Or at least a minimum) which would be nice to start out with, but they cripple them by allowing you to make only 6 web pages unless you pay like an additional $8 a month.  Kinda ridiculous just to post a couple of things, but I am still thinking of going for a year for the most basic plan just to learn a few new things.

Anyway, this is all off topic.  I don't want to turn into one of those couple of top "Top Participants" that constantly turn these threads into multi page unrelated ramblings about all the social woes in the world. That is why I rarely com on this forum and mostly use the raspi foundation forum.