voltage reference – Experimental Engineering

This detector has now been retired from service since it’s a fair bit out of date. So here’s the teardown!

Unlike older detectors, this unit has a built in battery that never needs replacing during the life of the sensor, so once the unit reaches it’s expiry date it’s just trashed as a whole.

4 screws hold the cover on, here’s the internals of the detector. There’s a 3v CR123A LiMnO² cell at the right for power, rated at 1500mAh. A 7 year life is quite remarkable on a single cell!
The sensor is just to the left of the lithium cell, and is of quite unusual construction. Previous CO sensor cells I’ve seen have been small cylinders with a pair of brass pins. This one appears to use a conductive plastic as the connections. These sensors contain H²SO⁴ so they’re a bit hazardous to open.
There are no manufacturer markings on the sensor & I’ve not been able to find any similarly shaped devices, so I’m unsure of it’s specifications.
The alarm sounder is on the left, the usual Piezo disc with a resonator to increase the loudness.

The brains of the device are provided by a Microchip PIC16F914 microcontroller. This is a fairly advanced device, with many onboard features, and NanoWatt™ technology, standby power consumption is <100nA according to Microchip’s Datasheet. This would explain the incredible battery life.
The choke just at the right edge of the photo is actually a transformer to drive the Piezo sounder at high voltage.

Here’s the PCB with the LCD frame removed. Not much to see on the this side, the silence/test button top right & the front end for the sensor.

Here’s a closer look at the front end for the CO sensor cell itself. I haven’t been able to decode the SMT markings on the SOT packages, but I’m guessing that there’s a pair of OpAmps & a voltage reference.

After watching a video over at Scullcom Hobby Electronics on YouTube, I figured I’d build one of these precision references to calibrate my multimeters.

It’s based around a REF102P 10v precision reference & an INA105P precision unity gain differential amplifier.

For full information, check out the video, I won’t go into the details here, just my particular circuit & PCB layout.

In the video, Veroboard is used. I’m not too fond of the stuff personally. I find it far too easy to make mistakes & it never quite looks good enough. To this end I have spun a board in Eagle, as usual.

Here’s the schematic layout, the same as is in the video.

As usual, the Eagle CAD layout files can be found at the bottom of the post.

And the associated PCB layout. I have added the option to be able to tweak the output, to get a more accurate calibration, which can be added by connecting JP1 on the PCB.

As in the original build, this unit uses pre-built DC-DC converter & Li-Ion charger modules. A handy Eagle library can be found online for these parts.
I have however left off the battery monitor section of the circuit, since I plan to use a protected lithium cell for power. This also allowed me to keep the board size down, & use a single sided layout.

Here’s the track layout ready to iron onto the copper clad board. I use the popular toner transfer system with special paper from eBay, this stuff has a coating that allows the toner to easily be transferred to the PCB without having to mess about with soaking in water & scraping paper off.

Here’s the paper having just been ironed onto the copper. After waiting for the board to cool off the paper is peeled off, leaving just the toner on the PCB.

PCB just out of the etch tank, drilled & with the solder pins for the modules installed. Only one issue with the transfer, in the bottom left corner of the board is visible, a very small section of copper was over etched.
This is easily fixed with a small piece of wire.

Main components populated. The DC-DC converter is set at 24v output, which the linear regulator then drops down to the +15v rail for the reference IC. The linear section of the regulator, along with the LC filter on the output of the switching regulator produce a low-ripple supply.

Here’s the scope reading the AC ripple on the output of the DC-DC converter. Scale is 100mV/Div. Roughly 150mV of ripple is riding on top of the DC rail.

And here’s the output from the linear regulator, scale of 50mV/Div. Ripple has been reduced to ~15mV for the reference IC.
In total the circuit as built has a power consumption of ~0.5W, most of which is being dissipated as heat in the linear part of the PSU.

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Tag: voltage reference

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