These units are used to broadcast local audio, such as from a public address system or local microphone. They accomplish this by producing a modulated magnetic field that a hearing aid is capable of picking up.
Not many controls on this bit of equipment. A bi-colour LED for status indications, a microphone, external audio input, charging input & a power switch.
Popping the cover off reveals a small lead-acid battery, 2.1Ah at 12v. This is used when the loop is unplugged.
Here’s the main PCB, which takes care of the audio & battery charging. The inductive loop itself is just visible as the tape-covered wire bundle around the edge of the casing.
Here’s the input section of the main PCB. The microphone input is handled by a SSM2166 front-end preamplifier from Analog Devices.
This audio is then fed into a TDA2003 10W Mono Power Amplifier IC, which directly drives the induction coil as if it were a speaker. Any suitable receiving coil & amplifier can then receive the signal & change it back into audio.
With a recent order from a Chinese seller on eBay, this little gadget was included in the package as a freebie:
I’ve not smoked for a long time, so I’m not too sure what use I’m going to find for this device, but it’s an electronic lighter!
Pushing the slider forward reveals a red-hot heater, mounted in the plastic (!) frame.
Pushing the other way reveals a USB port to charge the internal battery.
A couple of screws releases the end cap from the cover & the entire core unit slides out. Like all Chinese toys it’s made of the cheapest plastic imaginable, not such a good thing when heat is involved.
The element itself is a simple coil of Nichrome wire, crimped to a pair of brass terminals. The base the heater & it’s terminals are mounted to is actually ceramic – the surround though that this ceramic pill clips into is just the same cheap plastic. Luckily, the element only remains on for a few seconds on each button push, there’s no way to keep it on & start an in-pocket fire, as far as I can see.
The main PCB clips out of the back of the core frame, the large pair of tinned pads on the left connect to the heater, the control IC has no numbering of any kind, but considering the behaviour of the device it’s most likely a standard eCig control IC.
The other side of the board has the USB port on the right, the Lithium Polymer cell in the centre, and the power button on the left. The cell itself also has no marking, but I’m guessing a couple hundred mAh from the physical size.
This is the Velleman MK179 Proximity Card Reader, which is supplied in kit form. In the image above you can see the completed kit, the read coil is etched onto the black PCB on the left. Bringing a recognised card close to the coil operates the relay on the main PCB for a programmable amount of time.
Closeup of the main PCB, 12v DC input at top right. Left IC is an LM358 dual Op-Amp, the IC on the right is a PIC12F629 with Velleman’s custom firmware.
Logic power is supplied to the ICs & the oscillator from the LM7805 regulator at the top of the PCB. The relay is a standard 15A SPDT 12v coil relay, with the switch contacts broken out onto the screw terminals on the left.
As it is not provided with the kit, unlike other Velleman kits, here is the schematic for this.
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