MCU to turn On/Off a Bluetooth Module. - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: May 25, 2012, 11:21:02 am
content: Hi, I've been working on a home project for a good while now and I recently added a TTL bluetooth module to it, the communication side works fine but I'd like turn the Module on/off when required. The module is powered with 3.3v @ ~50 mA.Is this how I should do it?Thanks in advance

author: deephaven
author_rank: Frequent Contributor
timestamp: May 25, 2012, 11:30:11 am
content: Relying on the resistors to drop the voltage won't give you a very stable or accurate 3.3V. I would suggest using a low power linear regulator with an enable input.

author: Unknown
author_rank: Unknown
timestamp: May 25, 2012, 02:41:26 pm
content: QuoteIf I was already using a 7805 to power my system would I still need to add the linear regulator?

author: deephaven
author_rank: Frequent Contributor
timestamp: May 25, 2012, 02:48:14 pm
content: Quote from: jucole on May 25, 2012, 02:41:26 pmYes, the problem with just using a resistive divider like you've show is that it will be a relatively high impedance source of 3.3V to the Bluetooth module. This means that your nominally 3.3V will wave around depending on the current being drawn by the module. There are likely to be large variations in current drawn depending on whether or not it's transmitting etc.

author: Unknown
author_rank: Unknown
timestamp: May 25, 2012, 06:45:53 pm
content: Thanks for your reply; I was wondering is there any chance I could get away with a 3.3 Zener?like...

author: deephaven
author_rank: Frequent Contributor
timestamp: May 25, 2012, 09:46:01 pm
content: A mosfet would work better. Even then, this circuit wastes a lot of power by passing so much current through the zener diode. My first suggestion would be better.

author: Unknown
author_rank: Unknown
timestamp: May 25, 2012, 11:43:39 pm
content: Hi,What type of "low power linear regulator with an enable input" would you recommend for something like this?My only requirement is it be cheap, easy to buy and available in SMD.Regards

author: T4P
author_rank: Super Contributor
timestamp: May 26, 2012, 03:31:40 am
content: Quote from: jucole on May 25, 2012, 11:43:39 pmUH ? OF course it's the 78L33 ( smd version in SO-8 )

author: Unknown
author_rank: Unknown
timestamp: May 27, 2012, 09:55:09 am
content: better?

author: T4P
author_rank: Super Contributor
timestamp: May 27, 2012, 03:09:24 pm
content: Quote from: jucole on May 27, 2012, 09:55:09 amyes much better

author: Unknown
author_rank: Unknown
timestamp: May 27, 2012, 03:42:30 pm
content: You know some days I feel like my electronics moves just like a clockwork mouse, bouncing off chair legs and going around in circles, then every once in a while I get picked up and pointed in the right direction, and then off I go again ;-)many thanks!!

author: deephaven
author_rank: Frequent Contributor
timestamp: May 27, 2012, 06:08:30 pm
content: I would recommend the NCP511SN33T1GIt is available from Farnell (Element 14) order code: 1652367It is:Low powerLow drop-out voltageEnable pinSurface mountLow cost at £0.51

author: Unknown
author_rank: Unknown
timestamp: July 08, 2012, 08:43:41 pm
content: hi, i finally moved my project off the breadboard onto a veroboard for more testing; the Bluetooth is using a lm317 for this one as it was all i had in my boxes but my finished version will use one of the devices suggested above. many thanks!

author: Unknown
author_rank: Unknown
timestamp: July 08, 2012, 09:50:54 pm
content: Quote from: DaveXRT on May 26, 2012, 03:31:40 amDropout voltage is 1.7 V typical at Io = 40 mA and Tj = 25°C, worse at lower temperatures, higher output current or if your particular part is worse than typical. Assuming the enable signal from the MCU is 5V and not TTL as indicated, the emitter of the enable transistor will be at approximately 4.4 V, too low for even a typical 78L33 to maintain regulation at 50 mA.Get a proper LDO if you need it to work with a 5V input. While you're at it, you might as well get one with an enable pin so you don't need the extra transistor. The part deephaven suggests sounds fine, but I'm sure you'll find tons of matches with the parametric search on the website of any of the major distributors.

author: Psi
author_rank: Super Contributor
timestamp: July 09, 2012, 02:25:36 am
content: This solution wouldn't work for you, as you need more current, but sometimes you see low power 3.3V devices that only require 20mA or so and you can power them off an I/O pin.eg.. output pin ---> 3.3v reg --> deviceIt's easy to wire up, doesn't require any switching transistor and gives you control over the device.You *might* be able to parallel up more than one pin to get more current but that's not good practice.

author: Unknown
author_rank: Unknown
timestamp: July 09, 2012, 06:01:07 pm
content: QuoteI didn't realise that; luckily for me I have a 15v line as well as 5v which i fed to my LM317 to get my 3.3v, so the 78L33 should be fine, i got a reel of transistors from Ebay cheap (1.9 p each) so I'll probably use those instead of the uln2003 on the final pcb.QuoteSorry, if my "TTL" label on my drawing was misleading, i thought the output from my pic micro was at TTL, oh well.Anyway I fired up the controller last night and threw it some Bluetooth commands from my PC and bingo , it worked a treat!! so it's downhill now all the way! with just some spit 'n 'polish needed on the firmware!! :-)

author: Short Circuit
author_rank: Frequent Contributor
timestamp: July 09, 2012, 07:19:06 pm
content: Quote from: jucole on July 09, 2012, 06:01:07 pmWon't work, the emitter of the transistor will never ho higher than the base voltage - Vbe, regardless of collector voltage.Do yourself a favor and use a 'real' LDO with an enable input. One tiny SOT23/5, maybe 25 cents, and you're done.