Home > Uncategorized > Pocketalker PKT D1 teardown

Pocketalker PKT D1 teardown

October 2nd, 2015

Pocketeller has 3 screws and is very simple to open. There is really no precaution you need to take to open - it has two polycarbonate plastic enclosures and you see a Made in China sticker inside it. There are two black knobs that can be removed with two gold color screws revealing the potentiometers on which these knobs sits.

pocketeller1

The PCB is a very simple two layer PCB and only top side has the components. There is no component on the bottom side. And there are exactly two 8 Pin ICs.

TI LM4808

After looking at several headphone amplifiers as choice for the headphone, I was pleasantly surprised by the choice. The main reason of the choice of LM4808 is its low power consumtoin - 1 mA at 3.3V to be accurate as per the datasheet of LM4808. The other OP Amp I could find was TPA6100 rated at 0.75 mA ( @3.6V), but it has lower output power rating at 50 mW compared to 108 mW of LM4808.

The OP Amp itself is simple and consists of two of these. I did not dig deep into the components around it, but it looks like it cascaded the two OP Amps to get you the required tone variation with the external knob potentiometer.

pocketeller2

LMC555 When I first looked at 555 timer I was puzzled as to why the heck they are using the timer ? Then I realized that it is sitting next to the LED and immediately it clicked to me that it is for the ON / OFF control of blinking LED. It was then that I read the datasheet that said that the LED blinks when there is under voltage detection. So essentially, there is a under voltage detection circuit that triggers the timer which in turn makes the LED blink.

That brings me to my next investigation of battery life.

Current Consumption

Although pocketalker has a decent 100 hours of advertised life, I was still curious to know its current consumption. I took a multimeter and hooked it on one of the ends of the battery and measured the current under ON condition and with some sound on Microphone with Headphone connected. Here are the results ( The battery voltage was 3.12 V at the time of measurement).

S No Condition Current
1 No Headphone 3.52 mA
2 Headphone ON medium audio 20 mA
3 Headphone ON full audio 40 mA

The first row gives the most important result, it is the current consumption under no load condition. The second one is one the headphone is connected and you speak at a normal level so that the headphone speakers are moderately loaded. This figure is not very accurate and should be taken with a grain of salt. If you speak loud it will increase. At the peak, it takes 40 mA current. So while there is something audible, the pocketalker is going to take current somewhere between 5 mA to 30 mA. I should not be very far from real number, if I conclude that nominally it takes about 10 mA when listening something audible.

But the more important figure is the unloaded current consumption, as this is what is going to drive the battery life. The AAA batteries are rated typically at 1000 mAH ( milli Ampere hour). To find the number of hours of operation we divide the mAH number by the current which brings us to 1000 mAH / 3.52 = 282 Hours. Since the current consumption is much higher at the time when the headphone is loaded, the actual number should be somewhere in the range of 150 Hours. This is not far from the 100 hours of battery life as told by Williams sound.

I started digging a bit more and removed the LED. The current dropped to 2.80 V - a significant reduction. You could expect 20 % battery life improvement, if LED was removed. There is really little you could do with 2K resistor is series with LED, unless you want to either toggle the LED ON OFF with smaller ON period and a longer OFF period.

The other improvement could come from 555 timer. When I disconnected its VDD pin ( and with LED connected), the current dropped to 2.88 mA.

So in between the LED and the 555 times, approximately 40% power is wasted. We could possibly use a ultra low power microcontroller ( in microampere range) to toggle a bicolor green LED ON OFF in power on condition and RED ON/ OFF in battery low condition.

Uncategorized

  1. No comments yet.
  1. No trackbacks yet.