BS2pe doing too much?

Michael @ Afineol

I am concerned that I have asked my BS2pe to do too much. I have populated it with:

(1) 74HC165 Parallel to Serial - driving 5 switches
(1) 74HC595 Serial to Parallel - driving 5 Green LEDs each on seperate pins and only 1 at a time is active
(1) DS1302 Timekeeping Chip
(1) Parallax 2x16 Serial LCD (Backlit)
(1) Memory Stick Datalogger
(1) Piezo Speaker

It is working, but I have read too many posts that say my BS2 no longer works after I added ...

I would also like to add:

• another 74HC165 in series with the other 74HC165
• drive 2 or 3 Green LEDs·on each pin for a total of 10 or 15 LEDs again only one "set" of LEDs is active at a time

Any help would be greatly appreciated,

Mike

Mike Green

There are two issues here:

1) Can a Stamp, like the BS2pe, do a certain amount of work, like control several strings of Parallel to Serial and Serial to Parallel registers, a RTC chip, and a serially controlled USB host (the datalogger), etc?

2) How much power does the above set of stuff take?

Controlling a variety of devices with logic level inputs takes relatively little power. Driving relays or LEDs is a different issue. Supplying power to these devices is a different issue.

It may take too long to do the computations involved in controlling these devices for your purposes. In that case, you may need to get a faster controller or simplify what you're trying to do. I don't think that's what you're asking.

The Stamps have a small voltage regulator that's part of the module. It can supply maybe 50mA for the Stamp module itself and for the various I/O pins and the circuitry connected to them. Normally 50mA is more than enough for this with a bit extra thrown in. For example, a Stamp could easily power a single LED or the DS1302. It could power the 74HC165, but not the 74HC595 because it's powering the LEDs.

If you have the Stamp mounted on a board with a built-in regulator like the Board of Education, the board's regulator can power the other circuitry. These external regulators generally can provide 1 - 1.5A with a 6 to 7.5V input. A higher input voltage will cause more heat dissipation and, due to overheating, may limit the output current.

The general rule is "look on the datasheets". They give the maximum current drain. Add that up and compare that to what you have available. Start doing that and you will be able to answer this sort of question by yourself. The datasheets are all available with a Google search. Whenever you buy something, keep a copy of the file for the datasheet so you can refer to it later.

Michael @ Afineol

Mike,

Thank you for your post. I unfortunately do not have the background in electronics to perform the math required for my project. I looked up the datasheets as you suggested and filled in some of the blanks below, but as I said I do not have the skillset to determine the 74HC165 by itself or with an additional 74HC165,·74HC595 with 5, 10 or 15 LEDs (in sets of 1, 2 or 3) and the Piezo Speaker. Unfortunately, I am asking others to do my homework for me because I have enrolled in a "class" that I don't meet the prerequisites. Any help would be greatly appreciated either in a solution or a book I can read to arrive at a·solution.

(1) 74HC165 Parallel to Serial - driving 5 switches - A or V ?
(1) 74HC595 Serial to Parallel - driving 5 Green LEDs each on separate pins and only 1 at a time is active - A or V ?
(1) DS1302 Timekeeping Chip - 300nA @ 2.0V
(1) Parallax 2x16 Serial LCD (Backlit) - 80 mA @ 5 VDC
(1) Memory Stick Datalogger - 25 mA @ 5 VDC
(1) Piezo Speaker - A or V ?

Mike Green

The 74HC165 itself won't draw more than a mA. If you use a 10K resistor in series with the switches, that's less than 1mA each. The 74HC595 itself won't draw more than 1 mA, but the LEDs will take about 20-25 mA each (if you use a 220 to 330 ohm series resistor) assuming all of them are on. Add this all up and you get about 250mA (very roughly). That's well within the capacity of most external regulators