Air blade hand dryer - Aike GS-2006H

[Cogitosus]

Hi folks,

I noticed a fault on this unit in my local coffee shop, so I offered to fix it. After all I've lots of experience, how hard could it be?

**sigh**

So the issue is that when the dryer is first powered on it runs immediately and the timer counts down from 10 seconds to 0. However the dryer continues to run for about 10 seconds more and then shuts down with a flashing error LED and displaying E2, which means "Take your hands out of the dryer!"

It seemed so simple. After all it's just a motor, a heater and s timer, the whole thing triggered by an infra-red LED and a photo-transistor. Unfortunately I opened up the electronics and found this:

Sorry, for some reason I can't seem to embed a Flickr image, so please forgive the attachment.
Basically there's a helluva lot of stuff in there. The power semiconductors on the heatsink drive the DC brushless motor at a quite spectacular speed of 30,00+ RPM (I believe). The rest of the kits seems seriously over-complex for what the machine does, but the build quality is very high. Unfortunately that means that the entire board is lacquered on both sides to prevent corrosion.

The hand detector circuit is quite simple. It consists of four IR LEDS in series at the top front of the hand slot and three photodiodes on another (very tedious to remove) board at the lower rear of the slot. The photodiodes seem to be similar to **broken link removed** . They appear to be connected between 12V and eventually to the non-inverting inputs of an SM LM324 as shown in the .png file attached. I haven't had much luck tracing the circuit fully due to the dense packing of the SMDs.

I'm struggling at this point because my oscilloscope is on the blink and the IR has to be modulated to avoid daylight, so I presumably can't simply short the photodiodes to simulate full illumination.

It's a long shot, but does anyone have any experience of these or similar circuits?

 

[KeepItSimpleStupid]

Have you though of the possibility that IR/detector work like a reflective object sensor. The hand reflects the IR beam back.

 

[Cogitosus]

Indeed I did; in fact I was rather expecting that. However when i opened it up I found there was a transmit board and a receive board. Curiously, in addition to the three photodiodes, the receive board also has four LEDs for general illumination of the hand slot.

I'd expect the IR to operate at 38kHz, but I do have a slightly questionable and very cheap PC-based scope attachment and that seems to show about 20kHz on the LEDs. Viewing the IR emitters with a phone camera shows a dim glow. I daresay a good step would be to change the transmit LEDs.

 

[jpanhalt]

With capacitor coupling to the comparartor, the IR is probably chopped but not modulated like, say a Vishay unit is. That frequency is not going to be too critical, unless there is a high-pass filter somewhere that you haven't found. The 38 kHz is characteristic of units what have filtering. There would be no reason to have that frequency with what you have shown. As a source, common fluorescent bulbs emit pulsed IR.

John

 

[Cogitosus]

Thanks John, that's comprehensive. Since most of these dryers will always live under artificial light, wouldn't the circuit be filtered to avoid potential noise from fluorescent light ? If I could track down the 'decision' part of the circuit I'd be happy to fake it by adding a simple beam-break circuit and ignoring the existing bits, but it's really difficult to find my way around the SMDs and the lacquer makes it tiring trying to bell it out.

 

[jpanhalt]

That comparator sets the threshold. Thus, the intensity of the on-board IRED's should swamp what reflected IR gets to the device. You can see the IRED's with the view finder of any digital camera. Take a look at it and see if they light up. If so, then I would check the comparator output and see if it responds. After that, then you might be stuck because the rest of the circuit is obscured by the goo.

If the problem is that it doesn't turn on, then I would try to find the gate driver(s) for the power mosfets.

John

 

[Cogitosus]

Hi John,

I'm not sure exactly what I should be able to see with the camera. I've compared it to three random remote controls. A Samsung TV remote with a filter gives a bright purple light. A Sony remote with the filter missing gives a fainter purple glow, while a cable tuner remote with a very good range shows absolutely nothing through the filter when viewed with a digital camera.

The three IR emitters glow dimly, so there may be an issue with the drive circuit, or perhaps they are weak (equally?). Unfortunately the gate drivers aren't really an option as there are six of them, driven at high frequency. I think I need to change the IR emitters and monitor the comparators to see if I can spot anything happening. Perhaps a few remote controls shining on the photodiodes would help, assuming the 38kHz isn't an issue. I reckon I should have about 3.75V DC at the voltage divider under dark conditions, so I'll start there.