PDA

View Full Version : constant low voltage a concern?



cwyuzik
11-17-2010, 07:50 PM
Hi,
Just an inventor here considering using the propeller in an application that if feasable would use thousands of these chips. So far so good, fantastic product. I do have a concern about my circuit designs effect on the chip in the long range though.

The application requires that the device will only need to be used sporadically. So the program needs to shut everything down after a period of inactivity automatically.

Its using a 2x16 lcd for output and a qtouch capacitive circuit for a keypad as input. Since the keypad circuit uses so little current, it can be live 100% of the time as a method of input to start the application up when needed, thatís no problem.

Iíve devised a simple start circuit using a couple of transistors as switches. Holding down one of the qtouch keys for 2 seconds fires the base of one transistor enough for the prop to boot. Then once booted, one of the props pins latches the other transistors base to keep the prop and the lcd powered up. Once no longer required, the program can simply shut the device down by turning that same pin off.

Since the lcd and prop run on different voltages, the transistors work on the ground side of the supply.

This all works perfectly, however since transistors still conduct a little when no voltage is applied to the base, there is 2.14 volts to the prop and lcd in the off state.

In the manual it states the minimum voltage is 2.7 volts so as far as Iím aware, the chip isnít running. But Iím concerned the constant low voltage across vss and vdd may cause it harm in the long run. Iím hoping for the design life of device to be 15 to 20 years at least.

Is this circuit okay? I know that using a relay configured as a latch would be better, but cost is a factor.

My apologies if this issue has already been addressed. I couldnít find it anywhere.

Thanks for your anticipated opinions,
Clarence

Sapieha
11-17-2010, 08:02 PM
Hi cwyuzik.

Why not use small LVC device with Tristate output that have Output inactive on sleep.




Hi,
Just an inventor here considering using the propeller in an application that if feasable would use thousands of these chips. So far so good, fantastic product. I do have a concern about my circuit designs effect on the chip in the long range though.

The application requires that the device will only need to be used sporadically. So the program needs to shut everything down after a period of inactivity automatically.

Its using a 2x16 lcd for output and a qtouch capacitive circuit for a keypad as input. Since the keypad circuit uses so little current, it can be live 100% of the time as a method of input to start the application up when needed, thatís no problem.

Iíve devised a simple start circuit using a couple of transistors as switches. Holding down one of the qtouch keys for 2 seconds fires the base of one transistor enough for the prop to boot. Then once booted, one of the props pins latches the other transistors base to keep the prop and the lcd powered up. Once no longer required, the program can simply shut the device down by turning that same pin off.

Since the lcd and prop run on different voltages, the transistors work on the ground side of the supply.

This all works perfectly, however since transistors still conduct a little when no voltage is applied to the base, there is 2.14 volts to the prop and lcd in the off state.

In the manual it states the minimum voltage is 2.7 volts so as far as Iím aware, the chip isnít running. But Iím concerned the constant low voltage across vss and vdd may cause it harm in the long run. Iím hoping for the design life of device to be 15 to 20 years at least.

Is this circuit okay? I know that using a relay configured as a latch would be better, but cost is a factor.

My apologies if this issue has already been addressed. I couldnít find it anywhere.

Thanks for your anticipated opinions,
Clarence

Miner_with_a_PIC
11-17-2010, 09:21 PM
The Propeller is on (i.e. drawing some current) if it has ANY voltage applied to the rails, which blocks and at what speed they will function is questionable.

Have you considered using a voltage regulator with an enable pin in your design? Sending a high or low (depending on the part) signal to the enable will allow you to switch on the prop without off-state bias concerns.

Beau Schwabe
11-17-2010, 09:57 PM
cwyuzik,

I don't know if you would be able to apply something like this or not...

http://forums.parallax.com/showpost.php?p=730860&postcount=7

...Originally this was designed using a BS2 on a doorbell for my daughters playhouse. A 9V battery would last about 5 Months using this circuit.

However, It sounds like you might have a current path leakage problem that should be addressed first. Can you post any schematics of your setup?

cwyuzik
11-18-2010, 11:58 PM
thank you all so much for your suggestions and input on this. You guys do a good thing here. Not being in this feild I wasn't aware these components even existed.

At any rate, I'll order some of these in and do some research and testing. Once I get it working I'll post a scematic here for posterity sake.