Basic Stamp 1 Project Board Driving Capabilities

jukingeo

Hello All,

I have been messing around with the Basic Stamp 2 for a while now and I am finding out that for most of my projects the BS1 is sufficient for most tasks. Furthermore I see Parallax offers the BS1 Project Board for around $15. I think this is a good deal for a board that comes with power supply connections, a serial interface connection AND a breadboard section. However, the trouble is that the outputs can only source/sink about 20ma and the whole IC can only do about 40ma. The trouble with this is that for a full output on situations (all 8 outputs on) that would only yield 5ma per output. I find this very limiting...so much so, that I am wondering if 5ma is enough to even light an LED.

I know that LED's generally have a max current of about 20ma, and I have limited current to 10ma on an LED with good results, but 5ma??? Does anyone know the minimum current needed to fire an LED?

Then this brings me to the next question. With a 5ma simultanious output limit this rules out conventional relays and I am not even sure if this would be enough current to drive a transistor driven relay (unless perhaps a FET is used, which should draw practically no current from the control gate). But this would add considerably to the complexity and cost of the project.

I am not sure if 5ma is enough to drive a solid state relay. I could go this route, but I am afraid that using solid state relays can get very expensive.

I am open to suggestions on what I can do that would be cost effective to either boost the 40ma total current capacity of the project board, or perhaps if there is a low ma drive LED solid state relay out there that is affordable which I can use.

Thanks in advance for any advice.

Geo

search

You can use an external voltage regulator chip, to power both BS1 and your peripherals. I use the MAX603 regulator, and I posted a circuit diagram lately in the Completed projects category.

You can also use a transistor with a high ohm input (1kohm for example) to drive relays and LEDs, I have done that in my projects. It would also be possible to use an OpAmp with an input that uses only milliamperes to drive your peripherals.

Either way, you will have to include a Voltage regulator chip to drive the things you connect. A voltage regulator powering the BS1 and peripherals driven by transistiors is what I would do.

With the MAX603 you can use up to 500mA.

Here is an example using the MAX603 voltage regulator and transistor to drive a LED:

Post Edited (search) : 2/12/2008 8:11:48 PM GMT

Bruce Bates

Geo -

You may wish to take another look at the BS-1 pin port ampacities. It all depends on how your load is spread out, but you can certainly draw more than 5 mA per pin port in most cases. Here is the basic layout:

Per Port Limit (All 8 Pin Ports)
40 mA source, 50 mA sink

Per PIN Port Limit (One individual pin port)
20 mA source, 25 mA sink

You are taking 40 mA / 8 (source) and saying you can only draw 5 mA which would only be true is all 8 pin ports were driven to their max within the Total Port Limit. Consider that two pin ports driving 20 mA each is still within both the Total Port Limit and single Pin Port Limit, and is a far cry from your 5 mA single pin port calculation.

Both limits must be kept in mind when calculating the total port ampacity for the BS-1 Stamp.

Regards,

Bruce Bates

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jukingeo

Hello search, Bruce:


Search:

Essentially what you were suggesting is running a second circuit to drive the relays via a larger regulator, but you need to drive each output with a transistor and resistor. As I said above, this is the 'middle' step I would like to avoid. With a full cap 8 channel output, that is 8 resistors and 8 transistors. Too much fuss and expense. If there is no way to avoid this, then I perhaps would be better of with solid state relays.

Now, I WOULD go this route if I can get those resistors and transistors on a single chip. That would keep costs down and everything would be on a nice neat IC package. I WAS looking into the ULA series of IC's...but it says the input current is 25ma, I need something between 3 and 5ma.

Bruce:

The situation of all 8 inputs being on is very real. My application is a light controller for a theatre marquee and there will be an 'all on' feature in the programming as well. So taking that fact into consideration, the BS1 cannot exceed a total current of 40ma. Simple math got me to the 5ma. I am not one to take circuit designs to the limit either, so more than likely I would think 4 or even 3ma is more ideal. But with all circuits on any more than 5ma per will cook the BS1.

All:

So that brings me back to my first question. How much current do you need to light an LED?

Thanx,

Geo

Mike Green

jukingeo,
The amount of current needed to light an LED depends on the specific LED and how bright you want it. Most typical LEDs are specified at 20mA, but will produce light at lower currents and will tolerate currents to 30 or 40mA before they run the risk of overheating or damage.
Some LEDs are designed for much higher currents, but they're specifically designed for that use.

Have a look at the Prop-1 from EFX-TEK (www.efx-tek.com/topics/prop-1.html). This is a BS1 with a ULN2803 Darlington driver and is designed for just this sort of use. I think there are schematics on their website and you could duplicate that part of the circuit. You still will need the LED current limiting resistors. The ULN2803's logic can be driven from the BS1's regulated 5V output and the LED's can be supplied from another power source. Look at the ULN2803 datasheet for information on it's use.

Post Edited (Mike Green) : 2/13/2008 5:10:20 PM GMT