Attiny85 hardware interrupt unreliable, 433 MHz remote

Question

SCROLL DOWN TO SEE THE SOLUTION (UPDATE II)

ORIGINAL POST: I am currently building a 433MHz remote with two buttons. The remote is controlled by an Attiny85v(8MHz). Sadly it does not work very reliable, sometimes it works other times nothing happens.

Here some explanations on what I did and why:

Each of the buttons triggers a pin change interrupt which wakes the Attiny from sleep. After that I check which button was pressed and turn on or off the lights via 433MHz switches. But those buttons don't work very reliable. Sometimes the do what they should other times I need to press them for several seconds before something happens. I don't know why, either I have a bug in my code or in the circuit for the buttons.

I use a 2.2k resistor to pull the button pins (0 and 1) to GND. When pressing the button, the pins will be pulled HIGH. To avoid "bouncing" of the tactile switch I added an 0.01uf capacitor in parallel.

I tried to keep the code simple and added comments. You can find it here: http://pastebin.com/hzw0j3FW

Here is my circuit:

Eagle schema:

Front & back:

UPDATE: Thanks for all the feedback, I also received some feedback on a german forum. Here is a list of what is/could be wrong with the design:

• Don't pull the pins HIGH when pressing the buttons. Instead use INPUT_PULLUP and pull then to GND when pressing.
• As pointed out here, as well as in the forums: Either use a software debouncing or an inverted Schmitt-Trigger
• When using software debouncing make sure to only set flags in the interrupt routine and act on those flags in the loop routine.
• Test if the RF-transmitter has enough juice
• Add an capacitor across VCC and GND of the Attiny
• Try powering the whole circuit with a lab power supply. Maybe the coin cell can not provide enough power.

Sadly I flashed my last Attiny85v to death today, so it will take some time for me to fix this design. I will report back once I have a solution.

UPDATE II, Solution: IT'S WORKING. The design is flawed but it works. Here are the two 'bugs' I found.

1. Use high quality switches. I sourced my switches from Farnell but they were of very bad quality. You had to press them down an then move your thumb to make sure they connect. This was the reason why the remote only worked spontaneously.

2. The second problem was that not all rc-power-plugs were turned off when pressing the off button. Sometimes it would turn off all three of them, sometimes only one. This was because of the rf transmitter. As @Michal Canecky and @Ricardo pointed out those transmitters run on 3V+. They do work with 'only' 3V but the range is very limited (2m in my case). By soldering an 15cm antenna to the transmitter I was able to get a range of 3-4m. Now all rc-power-plugs turn off reliably.

I also used a scope to test if the coin cell was not strong enough by measuring the voltage of the transmitter during transmission. As it turned out it was fine and constant.

Thanks again for the tips.

Still the design is flawed, here is what I would improve or do again if I were to rebuild it:

1. Don't pull the pins HIGH when pressing the buttons. Instead use INPUT_PULLUP and pull then to GND when pressing.

2. Keep the capacitor over the buttons for debouncing. I used a scope to test if it works, and the results were solid. Make sure to not go over 0.01uF.

3. Add a capacitor between VCC and GND of the Attiny

4. In the code: Make sure you disable interrupts after receiving one, this also serves as a software debouncing of the buttons. This was already reflected in the original code. Don't forget to turn on interrupts before going to sleep ;)

5. If you need a range greater than 3m, use a voltage doubler (thanks @Michal Canecky) or a power source with more than 3V. Use the Attiny to turn the voltage doubler off after transmission.

6. Add a capacitor parallel to the battery to improve battery life, especially for noname vendors. This is only needed if your circuits peak current is above 10mA. Have a look at this excellent article from TI. It also explains how to calculate the capacity.

This is what the code of this improved version would look like Code v2 (Note: I did not test this code)

Here the updated circuit that I would use:

Answer 1

I have build quite a few wireless sensors and remote controls based on those cheap 433Mhz modules and tiny85. Here are my observations:

You don't need to power the wireless module from the microcontroller pin in order to save power, it consumes near nothing when the data pin in low.

When the data pin is high (it transmits) the power consumption is about 14mA, so it consumes about 7mA on average to transmit a message consisting of ones and zeros.

You really need an antennae, just solder a straight 17.2cm wire to it. You can just loop it around the perimeter of the remote if you can't keep it straight.

3V power supply is quite low, when your battery goes down to 2V it's even worst. I use a charge pump voltage doubler in my design to power the transmitter. It's powered down in sleep mode, I turn it on before transmission and turn if off after.

The button cell battery can provide only few mA and if you use the voltage doubler you will also double the current drawn, so you need to use a larger cap on the output to accumulate enough energy for one transmission burst (you can't continuously transmit)

I made an RF library specifically for those transmitters https://github.com/cano64/ManchesterRF Check the example codes for transmitter and receiver.

About sleep mode. You can only wake up the tiny85 using hardware interrupt, and only when it goes LOW, tiny85 has only one HW interrupt pin (pin 7, PB2) so you can use only one button directly to wake it up and it must be pulled HIGH when open.

Here is how to use two buttons for wake up using two extra pins. Wire it like this: [Pin1] -- [btn1] -- [PB2], [Pin2] -- [btn2] -- [PB2] Before going to sleep: set Pin1 = LOW, Pin2 = LOW, so when you press any of them it will wake up the microcontroller. After Wake up: To determine which button is actually pressed, put Pin1 HIGH Pin2 LOW, and check the state on PB2, then switch, Pin1 LOW, Pin2 HIGH and check the state on PB2. It takes only a fraction of a second to wake up and the user will still be holding the button. PB2 must have internal pullup enabled or use external pullup

Answer 2

Without the transmitter part number or a link to the transmitter datasheet, some information will have to be assumed. Also, without the right gear and knowledge to test RF circuits (both too expensive), it's really hard to debug them. But we can always tinker with these circuits, right?.

I would guess that the problem is that the transmitter is not getting enough power to transmit reliably. These transmitters accept 3 to 12V supply, but the more the better (i.e. more power for transmitting). With your coin cell, you're giving it its minimum supply voltage (3V). At that voltage, the transmitter will use about 5 to 15mA, which may not be enough for it to work reliably.

One test that you may perform is to bypass the coin cell with a more powerful 5V supply (maybe from 3 1.5V AA alcaline batteries in series, or a bench power supply) and check if the remote control performs better.

Another issue may be the lack of an antenna. I can't see one in your pictures and also there's no link to the RF module antennae pin on your schematics. I'm no antenna expert nor I have the right gear to test that stuff (too expensive), but by trial and error, I found out that my 433MHz remotes only work reliably when equiped with a 10cm antenna. Without an antennae, my remote control would only work at a few centimeters from the receiver. I usually use a bit of those internal wires found in LAN cables very handy for that.