Hey guys!
I already managed to get my circuit working which follows the attached circuit schematic. I added a voltage divider (2x10kohms) and an output LED (just for indication purposes) at the midpoint of the divider at Q2 collector side, which is the output pin. I used the divider because the output is 9V and I should trim it down to atleast 3-5 volts to be more pin-friendly.
During normal circumstances, the LED (indicator) with the 470 ohm resistor at Q1 is off and the LED (output) at Q2 is high by default. Whenever smoke is detected, the LED connected to the 470 ohm resistor at Q1 turns on and the Q2 LED turns off. Everything seems to be working fine.
My problem comes when I attach the output (Q2 collector) to my Arduino Uno. Just removing the LED at the midpoint of the divider and connecting a jumper wire instead in its place, to the digital pin of Arduino, the indicator LED does not turn on or respond. There also seems to be no changes in the output state from HIGH (default) to LOW (smoke detect).
My program in the Arduino is simple. I assigned a pin as an input, read that pin for any signal (HIGH or LOW) and output anything if it reads high.
What is the problem? Is it because of the high impedance of the microcontroller, high capacitance or what?
Thank you!
Q2 is open collector, so no voltage will ever appear on the output. You need a pullup resistor on the output. Declaring the Arduino input as INPUT_PULLUP should work.
Transistors Q1 and Q2 are not matched, so they require separate base resistors. 1K or even 10K each would be more appropriate and not waste so much power.
Thanks jremington.
I have observed other things also about the circuit. Even on alarm condition, whenever I share a load with the battery (Li-ion 3.7V) that's powering the circuit (by the way it needs 9V, so I do battery -> adjustable boost converter (3.7 to 9V) -> detector)), which is the Arduino Pro Mini + some sensors + button + XBee (which also has a 0.9-5V boost converter), the smoke detector circuit is somehow behaving weirdly as if it's running low on battery or does not like to have anything connected to it except LEDs or resistors (which I only have right now). Whenever I disconnect any wires between the circuit and the Arduino, it starts behaving normally again.
I need to interface it with an Arduino, so with this problem, I'm kind of really stuck and I really lack experience with electronics, sadly.
The circuit uses very low current and wakes up every 1.66 seconds (single clock cycle). Other features are described in the datasheet. Is it because of the low current consumption and high impedance of the Arduino?
Thank you
A5368-Datasheet.pdf (171 KB)
The final circuit looks like the attached pic.
That isn't a circuit diagram. It is a block diagram and is useless for detecting circuit problems.
Smoke alarms run for years on a 9V block battery, so use one, and don't bother with the switching converter.
