Driving the vibe

[plumber]

Can somebody help me with a project?
For hearing impairment: I need to change a "beep" into a "vibration", and a analog meter needle into a rapidly flashing LED. This would allow my wife to enjoy the hobby of metal detecting, despite her hearing loss. I know very little about electronics, so please bear with me. Instead of a audio signal that gets higher pitched as the detector coil comes closer to a buried target, I need to build a wrist band or earlobe clamp that will vibrate when a buried target is detected. The vibration motor or device would need the ability to increase cycles or oscillations as the detector coil comes closer to a target. So ideally, the vibration would grow in intensity and decrease depending on signal strength. I would also like to convert the meter signal to a flashing LED...instead of a swinging needle. Ideally the LED would flash more rapidly as the target gets closer. A solidly lit LED would indicate dead center, or pin pointing of target. I know this is possible to do, under water detectors operate this way. But can it be done retro? and how complicated would it be. Even if this sounds like a stupid question or project, I would really appreciate any input at all. Or showing me the direction I need to take.
Thank you for your time, plumber

 

[(*steve*)]

Well, first you need to find your vibration device. The simple ones (used in mobile phones) are a very small motor with an offset weight which hauses them to shake when the motor turns. And it seems like you would want to change the amount of vibration to match the frequency of the sound you can't hear.

I would look at building a monostable that has a period which corresponds to the period of the highest frequency that is produced. If the high pitched sound is 2500 khz then the period would be 1/2500 sec = 0.4 ms. Then the output signal should be connected to something producing a very brief pulse on each incoming rising edge of the waveform. The output signal (suitably buffered) could be used to drive the motor which (via pulse position modulation) would now rotate (and hence vibrate) at a speed proportional to the frequency of the signal.

A simple way to make this would be to amplify the signal to ensure it generates a good rail to rail clipped square wave, then drive a couple of cmos schmitt triggers, one set up to generate a very narrow pulse, the other set to extend the pulse for 0.4 ms -- then to a transistor or mosfet to switch the motor using this signal.

The flashing LED is a little more difficult because it's hard to know what the meter is reading. I'll assume it is reading a voltage (say across a capacitor), but other alternatives include it being used to integrate a pulse width or pulse position digital signal. I would look at creating a voltage controlled current source to charge a capacitor, and use this with a 555 to generate pulses of a fixed length, so that the flashes of the LED get closer and closer together as the voltage rises (i.e. as the meter reading moves to the right).

The first option is pretty simple. You could build it completely out of CMOS logic (using a -- or a couple of -- inverters biased to operate as linear amplifiers followed by some schmitt triggers to do the pulse modification stuff, and then the remaining gates could be used to provide enough drive to operate the motor)

 

[plumber]

Well, first you need to find your vibration device.

Thank you (*steve*) ! I couldn't understand most of what you wrote, but it gives me a place to start studying. I don't know if it will change anything, or make things easier, but a few things came to mind. The LED display is much less important. It would tend to distract if relied upon too much. Maybe just as an initial visual indicator that the vibration indicator is about to, or has started to vibrate. So the operator could slow down and begin focusing on the vibration. The other thing is the vibration device. This device could be just about anything capable of creating varying physical pressure in the operators skin. A vibration may not even be the best way to accomplish this. The important thing would be the "distinctness" of the pressure changes. A ticking clock you could feel comes to mind. I think time intervals in between ticks or pressures would be more noticeable than the intensity of the pressure rising or dropping. So, if any gadgetry comes to mind, let me know. If not, sticking with a vibration device will get me going in the right direction. Again thank you very much for your time! I will begin studying towards applying your input.
plumber

 

[(*steve*)]

vibration is a fairly good non-auditory input. Light (from LEDs) is good, but I'm assuming that you may be using your metal detector in various lighting conditions, and a LED bright enough to see in the midday sun would be blinding at night.

Here is an example of a vibrating motor http://www.goldmine-elec-products.com/prodinfo.asp?number=G16777

you can vary the speed of these devices, and you could also have them pulse on an d off, with the pulses coming close and closer together. I'm not sure which would be most effective.