Trying to make an RF transmitter with 4 buttons and no standby power-consumption - Page 1
Forum section: electronics / beginners

author: Kim_Kristoffersen
author_rank: Newbie
timestamp: April 25, 2013, 12:30:41 pm
content: Hello hobbyists!I need some help in designig a circuit for an RF transmitter with 4 buttons(for car access++)In this circuit I have thought about using a microcontroller(Attiny85 or Attiny84) sending 4 accesscodes over RF.What i Really need help in understanding is how to make this circuit completely "dead" for lack of a better word,untill one of the 4 buttons are pushed.I also need help in making a very simple RF system for transmitting data up to about 50 meters.I want this circuit to be as small as possible, like a modern car-key, for this reason I want to use a CR2032(3v) battery.Any help will be greatly appriciated ;-)Kim KristoffersenSandefjord, NORWAY

author: psycho0815
author_rank: Regular Contributor
timestamp: April 25, 2013, 01:09:21 pm
content: My first thought would be to use a 2-pole button to switch both vcc and pull the appropiate µC-Pin to ground. Than just check for the pin on Startup in your firmware.Off course, that would require you to hold the button down until the signal is sent.Another idea would be to use a Transistor to switch the circuit on and use a pin on your µC to latch it until its done.

author: Codemonkey
author_rank: Regular Contributor
timestamp: April 25, 2013, 02:08:48 pm
content: I wouldn't bother trying to get the thing to be completely powered down. If you do, you'll need to either keep the button held down until the code is sent, or extra circuitry to ensure it stays powered if the button is released early (debounce issues).My solution would be to use something like this from NXP/Jennic:http://www.jennic.com/support/datasheets/jn5168_wireless_microcontroller_datasheetIts a 32 bit microcontroller with integrated 2.4GHz radio transciever and baseband. You can put it in a deep sleep mode whereby the thing only consumes 0.12uA and is woken by any of the DIO pins changing state and will run directly off a coin cell as long as you don't stay awake too long (they have apps note's demonstrating coin cell operation).To give you an idea of how efficient these things are, they have a demo where the action of pressing a switch generates enough energy to power the thing long enough to transmit a few messages to something to turn a lamp on or off (no battery required).It would be very simple to knock up an application that wakes up, transmits a packet with the codes in it (you could even use the on chip AES encryption engine to encrypt/decrypt the mesages on the fly), then go back to deep sleep and wait for another button press.So, basically a 1 chip solution to your application!I've been using their last generation JN5148 modules in applications for a while and had no issues. The eval kits are expensive but you don't really need one. All thats required is their SDK which is a free download and not code size limited or anthing (gcc based), and an FTDI USB-Serial cable to flash the device.

author: David_AVD
author_rank: Super Contributor
timestamp: April 25, 2013, 09:04:49 pm
content: Some remotes controls use diodes to make an OR function that powers up the chip, but keeps the button lines separate. The chip latches power on until it's finished sending the data.

author: Kim_Kristoffersen
author_rank: Newbie
timestamp: May 01, 2013, 10:45:50 pm
content: Thanks for all the replies guys, got some great ideas ;-)