Introduction: Hacking an Otherwise Dysfunctional Digital Recorder - Finished !
I want to show, not just one way to hack this device and make it more functional, but the process & steps I took to decide how and what parts to improve.
Thus, the $10-$11 investment made become more worthwhile (especially if one takes it "all the way".
So, let's get to making this device worthwhile.
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Step 1: Parts / Equipment N Cautions
Of course, one needs to have the circuit pictured in the Intro. It records a very LOW level of sound IF one gets within millimeters of the Speaker, which is also the acting mic.
Whichever "AMP" one wishes to use, or the one I will be building in a subsequent step (meaning the parts for it),
The Amp I constructed used:
One: LM386 audio amp
Three: 10uF polarized capacitors
One: Non-polarized 1 uF capacitor
One: 100 uF polarized capacitor
One: 1 nF ceramic capacitor
One: 47 nF ceramic capacitor
One: slide 10K pot; three legs
One: socket, for AMP
8 ohm 4 ohm speaker
A DPDT switch ( instead of having the mic draw power all the time, since output was already weak, I install a switch ), or 2 SPST switches.
and of course solder, soldering iron, eye protection, and caution working around the equipment.
The Amp schematics I used can be found here: Audio Amplifiers / simple LM386 Audio Amp and is similar to the one below, but not exactly the same.
Step 2: A Beginning. . .
When I got this "digital voice recorder" originally it was to use for my April Fools instructable. But it's recording capabilities were Nearly non-existent, and playback was impossibly low in volume.
My first thought was that, since the Mic and the speaker were the same (this can be done, but as we see, with poor results), I decided to add my own Mic.
But, after testing it, I found the sound only slightly improved. So once again, I decided to take some of the power users out of the equation and installed a switch in line with the Mic (had I had a better switch, I could have wired it as an on / off - off / on; that is, on for mic off for speaker OR on for speaker off for mic).
This too improved the quality of the recording, but not the volume. I was beginning to wonder if it was the chip and I would then have to give up on it for now. The chip I used 20 years or so ago, was not available from RS as they did not get much business anymore for "parts" of that sort.
Ok, well there is always amplification :-)
Step 3: The Pre-amp / Amp
In putting this together, I ran into a little difficulty locally finding a 10 ohm resistor. But, once acquired (yesterday - today's date is on the photo), it was pretty easy to finish up most of the way.
I have all the components in and it is ready for setting up and testing.
When I feel up to it, I will wire it all together and give it a go. First, I want to test it on the "back end", that is, as an amp for the meager output of the DVR/P chip.
I do believe, from my early experiments in attempting to up the volume input, that the chip was unable to handle the higher volume, so it would probably be a waste of time trying to up the volume being recorded.
Step 4: Final Revisions
I finally got the speaker off the RS device, and put the amp (gain of approximately 200) in between.
The first test shows me it worked, in a limited sense. The volume is way up, and I can actually hear the output, however there is a bit of distortion and maybe some clipping going on that I will have to tweak out later. I may need a better matched or more sensitive mic.
For now, this is a step in the right direction, and only (I hope) needs a bit of tweaking to be added as I have time.
I will publish this however as it is along overdue.
Step 5: The Pre-Amp
Although the amplification is good, the output is still quite rough.
Before trying to alter the circuit I have already installed, I thought I should try to clean up the input, since it was originally getting it from a speaker as a mic.
I added a pre-amp to help drive the mic's signal. This helped some again, but I still need to compensate for what sounds like a bit of overdriving the speaker.
I think I will try to find a better speaker and see if that doesn't help some.
Step 6: Couldn't Find a Suitable Speaker (I Have One, I Just Can't Find It at This Time :-)
So, just for grins and giggles, I replaced the Mic (it was a small "crystal" mic, I found out, and needed a bit more "drive" than even the pre-amp could supply).
It sounds a WHOLE lot better now. The speaker will make a big difference too, once I come across it. Then I will make it look nice (box it and such), and although it will not be as small as those units you can buy for reminders, this started out as no where near that small a device that was non-functional from new. And now it has function :-)
Step 7: And Now for a Change of Speaker
Below is the speaker and assembly I will install next, to see of the impedance response is better (better matched).
The voice quality was improved tremendously. Shunting the output with a .5uF non-polarized cap also seemed to smooth out some of the clipping that was coming from the 200X amp.
Step 8: Putting the Speaker Back Where It Belonged....
I was having a real Dickens of a time trying to find a suitable "container" or box for this project...then I realized that the enclosure that originally held the speaker I used, a small tower of plastic for what was 1/2 of a computer speaker pair, would work perfectly.
My only problem will be in incorporating a way to adjust the slider switch (volume), which I have soldered to the PC board. This may be a bit tricky. Here are some pictures to show where I am going with this:
Step 9: S.M. Buttons and Other Problems....
Handling the surface mount buttons, the 2 switches, and the PC board mounted sliding volume control are my last problems as well as finding a "place" for the battery.
I mounted the "speaker / mic" switch directly behind the ( record / play) board at the top of the mechanism. The "power, on / off" switch is just left of that (both out of view of these pictures).
I was having diffulties working with the plastic of this case, so I didn't have an elegant answer as to how to mount the board mounted push buttons. They are hot glued (for now) to the top of the speaker cabinet.
The second picture shows the slot access to the slider volume control. I need to arrange for a way to adjust that yet too.
More then likely, this whole project could have been 10 times easier if I had just used the components of the speaker (internal amp, volume controls, etc) BUT it would have two things against it.
#1: it would need a 12 vdc power source (transformer)
#2: I haven't built an AMP from the ground up for nearly 20 years, and I needed the practice :-)
Step 10: Finishing Up
Here is the final product. It is still a little crude, but I haven't the resources I once had.
Especially crude is the battery holder. I had nothing else in the house that would work as well. The Styrofoam was available.
The non-functioning boards are there for two reasons. On to help hide some of the wiring and the fact that the small "original" board (with the red LED on it) overhung the top a bit.
Adding the LED was difficult, as the tiny red one was so close to the board. First I had to determine which lead was the anode and which the cathode. An analog multimeter I had handy determined that fairly quickly (after I got 20 seconds of collaboration from my wife ;-).
I tinned my iron tip (so it had a little solder on it and would draw any from the board), touched the joint for a second or two, and pulled back, creating a small "peak". Then I tinned the wire leg of the "added" LED and then soldered it in.
This instructible is more of a "do this with what you can find around the house" kind of project. Except for the AMP, which I had already gotten parts for, everything else was scrounged from my junk box.
Here is how I finally got it to sound:
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