Minimum Hardware Requirement

Using Arduino Project Guidance
1

Hey.

I am new to Arduino but before I start I have a question about what motherboard/expansion modules I need or can use.
I have a simple but physically large system I'd like to automate with relays (with the potential to do more in the future).

The main requirement is the ability to control at least 15 relays.
If they are all PWM controlled I would need a board with at least 15 digital outputs that can be configured to be PWM. So I would need the mega2560.

The next main requirement is for the board to communicate with an external system via serial RS232. Would this require some sort of adapter/module? I want to send and receive ASCII/HEX codes to and from the Arduino. I see that it is "Serial" but it is also TTL which I am unfamiliar with.

Thanks for any help you can provide.

2

Why in the world would you want to control a relay using PWM. ???

What loads are the relays controlling?

Yes you need a TTL to RS232 converter if you need RS232 levels, lots on the internet.

3

Relays and PWM do not go well together. The chattering will be noisy, and they have a lifetime limit of how many contact closures they are rated for.

Using Analog.write(pin, level) with level from 0 to 255 will put out a square wave of varying width at ~488 Hz (cycles/second).
If the relay was rated for 1,000,000 cycles, then after 1000000 cycles/ 488 cycles/second = 2049 seconds it will be nearing the end of it life. (34 minutes)

To communicate via RS232 you need something with a MAX232 family of chip on it, such as
http://www.nkcelectronics.com/RS232-to-TTL-converter-board-DTE-with-Male-DB9-33V-to-5V_p_369.html

(forum may stick some extra characters on the start and end of that)

to convert the RS232 level of +/-3V to +/-12V (typical, could be as high as 25V) down to 0/5V for the Mega to use on Serial1, Serial2, or Serial3.
Serial (0, if you will) is setup to communicate with a PC via USB, so I wouldn't connect the RS232 module to that.

4

Great! That's all I need right now. You are right, it probably won't be PWM in the end.

Thanks again for taking the time to reply.

5

Actually a relay will not operate ‘at’ PWM frequencies.

6

CrossRoads:
Using Analog.write(pin, level) with level from 0 to 255 will put out a square wave of varying width at ~488 Hz (cycles/second).
If the relay was rated for 1,000,000 cycles, then after 1000000 cycles/ 488 cycles/second = 2049 seconds it will be nearing the end of it life. (34 minutes)

I don't think a relay will be able to physically react that fast. The induction of the coil will also seriously smooth out the current. It's more likely either on or off, depending on whether the resulting current is enough to activate the coil.

7

larryd:
Actually a relay will not operate ‘at’ PWM frequencies.

Certainly not the Ebay relays, but a bi-polar relay will operate that fast.

Paul

8

Yes, you can use relays with PWM. Your kitchen microwave does that, for example. When you select "low", "medium" or "high", a relay switches on & off the cyclotron using pwm. But the period of that pwm signal is measured in seconds rather than milliseconds or microseconds. If the cyclotron is switched on for 5s in each 10s period for "medium" power, that's just a 50% duty cycle pwm signal.

But periods of seconds are not what the Arduino community normally refers to as pwm. You can't do 5s in 10s with analogWrite(). Someone will prove me wrong about that, I bet! Some code to fiddle the AVR timer registers or something...

9

Well, by now we know that relays and PWM are not the first choice for.... well, we have no idea what you are doing.

Can you tell us what you want to control with the relays ? voltage, power and hopefully with a link to a data sheet.

Chances are there is some way to do what you want with common parts.

10

rOBvAN:
The main requirement is the ability to control at least 15 relays.
If they are all PWM controlled I would need a board with at least 15 digital outputs that can be configured to be PWM. So I would need the mega2560.

You have already gotten advice about not needing PWM, but 15 relays do not necessarily require a mega2560. A couple of shift registers will give you 16 digital outputs capable of driving the common optically-isolated relay boards, only requiring a couple of pins on the arduino.

The next main requirement is for the board to communicate with an external system via serial RS232.

That would be something that would tend to favor an arduino with a second hardware serial port (the mega has four, counting the USB interface). The UNO can implement a software serial port, but that has its limitations, particularly if you need a high baud rate.

11

PaulRB:
YBut periods of seconds are not what the Arduino community normally refers to as pwm. You can't do 5s in 10s with analogWrite(). Someone will prove me wrong about that, I bet! Some code to fiddle the AVR timer registers or something...

LOL you gave a challenge :slight_smile:
Timer1 16-bit so 65535 max value.
Highest prescaler f/1024 so at 16 MHz that's an overflow after just over 4 seconds. Meaning 10s to overflow is possible if you reduce the clock to <6 MHz or so. So yes, should be possible, but totally impractical and just fiddling the registers a bit is not enough.