First Alert Smoke Detector & Carbon Monoxide Alarm

First Alert Smoke Detector & Carbon Monoxide Alarm – I bought this smoke and carbon monoxide detector several years ago, and it’s been doing a great job so far: mostly silent, beeping when the toast burns, and beeping every couple of years or so when the battery dies. Lately though, it’s started making noises for no reason, and I can’t seem to get it to stop except by removing the battery. So I bought a new one and decided to get rid of the old one.

It’s made by First Alert, detects smoke and carbon dioxide, and has a test button you have to use once a week (seriously?). It runs on two AA batteries. First Alert is an American supplier of fire safety products currently owned by Resideo Technologies.

First Alert Smoke Detector & Carbon Monoxide Alarm

On the back there are some instructions for use as well as the date of manufacture and the “use by” date. In this case, the years are not published, so you have to look at your age.

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Inside is a single PCB. There’s a large white bezel on the right, and two sensors on the left.

Smoke sensors are optical type. The LED shines light in the dark room connected to the bottom of the PCB, and if smoke enters, the light is scattered and partially reflected on the photodiode. The exhaust chamber is made of black plastic and has a smart structure that limits outside light while still allowing air to flow.

Here’s a more detailed look at the optical sensor: the LED is an infrared type in clear purple plastic (left) and the photodiode for IR (right) is housed in a clear opaque package. The chip on the front drives the LED and reads the photodiode.

The brown cylinder on the back looks like a battery, but is actually a carbon monoxide sensor. This TGS5042 is made by Figaro and is based on the electrochemical principle: the small hole on the right side allows air to enter, the platinum electrode turns any CO into CO with oxygen.

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. The electrons sent in this process are captured by the electrodes and create a current proportional to the amount of CO in the air. At around 30 euros, the CO sensor is easily the most expensive part of this exhaust.

The hole is only visible on the edge here. All assemblies are basically the same size as AA batteries; One finds that these sensors are made using the same equipment used to make batteries. In fact, some rechargeable NiCd batteries have similar holes to release gases that may occur during charging.

The sensor is usually an open tube filled with a liquid electrolyte, in this case a mixture of potassium hydroxide and potassium carbonate. It is closed by a cover behind a small hole on the right side.

The surface is covered with metal, which is probably a platinum coated electrode. The ring on the right connects to the outer terminal of the sensor.

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In the optical sensor system, both such devices are mounted on a 5 mm LED package. On the left is an infrared LED in a clear purple housing with a cathode on the left and an anode on the right. Remember that in most LEDs the cathode is the larger of the two inner parts, but in this case it is slightly smaller; What does the flat side of the package tell you?

The photodiode is in dark plastic, so we can’t see what’s inside. This plastic is designed to be transparent to IR but opaque to other wavelengths, so the sensor is insensitive to anything but IR. The cathode is on the right side, which you can only tell from the flat side of the package (or from a test with a multimeter).

Note that both devices are basically diodes: the only difference is the material the junction is made of.

Inside the LED we find this tiny chip that measures only 0.3mm on each side. A single bond wire connects the cathode, which is the top surface of the chip. The lower part is attached to the anode pin, which is shaped like a cup: the light comes out from the edge of the chip, and the cup is reflective to wrap the light.

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Photodiode, meanwhile, is a rather large chip: it is 2 mm by 2 mm square. The chip design consists of only two layers, one p-type and one n-type, the upper layer is connected through a single gold bonding wire. The back side is directly connected to one of the metal pins. In general, the design of the package is similar to the LED, although the photodiode does not need a reflector because the sensor is not on the side of the die, but on the top surface.

The main microcontroller is the PIC16LF1937 microchip which coordinates all the functions in the smoke detector. There are some loose connectors on the corner of the PCB that are conveniently labeled to tell us what they do. The top has the standard PIC programming pins (MCLR, ICSP data and clock) and is also labeled “RS-232 OUT”. So I attached some pin headers and looked at them with an oscilloscope.

It is an active-high serial signal, 9600 bps. In ASCII, it starts with “{08 3”, so certain strings with numbers are obvious. There is a signal burst once per second; Let’s look at some information and see if it makes sense:

Most of these values ​​are constant, but some change over time: for example, the second column changes between 2A and 3A. Column 4 is usually zero but initially shows 1E and 2A. Similarly, column 6 shows five different values ​​in the middle of this log. In the second-to-last column of the meter that finally satisfies the FA when you open it long enough.

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I don’t know what the code means, but maybe it’s a status code that shows what the microcontroller is doing each time. Since the smoker must be used for a long time on one AA battery, it will probably perform all the functions once a second and stay in sleep mode the rest of the time.

Here’s a closer look at the processor. It’s a PIC16LF1937, which is an 8-bit ultra-low-cost microcontroller with 14KB of flash. It works at a voltage of up to 1.8 V and consumes only 60 nA in standby mode.

Here’s an overview of the PIC’s internals. Basically, we find analog circuits on the left, digital circuits in the middle and memories on the right. The size of the entire chip is approximately 2.3 mm by 2.6 mm.

Here we find the Microchip logo, although there is no copyright date. Notice how many lines there are at this 90 degree angle: they must all be different lines of force. It’s a bit odd that the chip only has one VDD and VSS pin instead of separate analog and digital.

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Next to the optical sensor, we find a chip labeled “I87-0205-000”. It is also equipped with a microchip, but the part number does not provide useful information. The second line has a date code (2013, week 24) and a microchip logo, but that’s about it. Microchip makes many exhaust and CO sensor chips, but the part numbers are not the same. However, the package is very similar, so this is probably a special version of one of the standard chips like RE46C191.

If we open it up, we find a very interesting collection of mostly analog circuits packed into a 1.9mm by 1.9mm square chip. It is made in a certain old process; We can clearly see the individual resistors and transistors. Two large power transistors dominate the upper left corner.

There is a die tag: RE265V02. Unfortunately, the numbers don’t help much: in fact, Google gives exactly zero on this article. There is a 2004 copyright date as well as the “R&E” logo. In the year R&E International, a small-scale semiconductor manufacturer, was acquired by Microchip in 2009. The name lives on in the ASIC smoke detectors that Microchip still offers. There is no model with the name “265”, so we will assume the exact chip we are looking at here.

We can clearly see the individual transistors here; Usually CMOS devices are used, but rare bipolar transistors are also used. Large resistors tell us that this is a low-power analog design, with a thick metal layer (the bright line running through the circuit) carrying a heavy current.

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Here is a close up of the output transistor. The RE series chips intended for optical smoke detectors contain boost switches that require large NMOS transistors. Another could be the LED driver.

Below zero we find this district. Two 8-pin chips are op amps, which are amplified.

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