Technology on a Small Scale
A Teardown
These days, you can find radios embedded in all kinds of devices. During a recent trip to the local Park and Swap, I found the the combination AM/FM radio & ball-point pen shown in Figure 1. For the wopping sum of $2 (brand new and in the box), I couldn't help but buy it for the express purpose of taking it apart to see how it was built.
This device was produced by a nameless company in the far east. Operation is pretty simple. There's a slide switch to select between AM/FM, along with a small tuning dial to select the channel of interest. Two LEDs, one above, and one below the tuning dial provide positive feedback as to whether we've selected AM or FM. The radio comes with a pair of ear pods, which connect via a jack in the top of the pen. Volume control and an on/off switch are inline with the ear pods.
Disassembly was done using only two tools, my trusty penknife and a small philips head screwdriver. In fact, the entire assembly has only one screw. Everything else snaps and/or slides together, and was easily taken apart again. All things considered, I found the mechanical design even more fascinating than the electrical.
The pen is divided into two primary subsystems. The lower half performs the pen function. There's nothing revolutionary here - twist to engage or disengage the pen.
Figure 3 shows the result of separating the top and bottom portions of the pen. Notice that the decorative metal band around the center of the pen is actually a machined insert. The narrow end wraps around the end of the ink cartridge in the lower portion of the pin. The upper end acts as one end of the battery compartment. Two 1.5V LR44 alkaline coin cells ($0.25 each in quantities of 1000) fit minus side down into the battery compartment in Figure 5. That then screws into the the insert - you can see the threads in Figure 6. The spring in Figure 5 makes contact with the minus side of the batteries. If you look closely, you can also see a blue-green wire near the spring. That makes contact with the metal insert, which connects to the positive end of the two batteries. Simple and functional.
A last note before moving on to the electronics. Figure 7 shows the detail you would see looking into the bottom half of the pen. This is the top of the ink cartridge. The narrow end of the metal insert slides over the ink cartridge with enough friction that a normal twisting motion is enough to engage the pen. The ink cartridge apparently is not meant to be replaceable. This product is clearly intended to have a brief lifespan!
The radio is in the upper half of the pen. Figure 8 zooms in on the tuning dial (shown again in Figure 9) and band selector switch. By prying under the tuning dial with my penknife, I was able to pop it off to reveal the only screw in the entire assembly (Figures 10 & 12). Next steps in the disassembly require my phillips head and (again) knife blade to pop off the plastic coupler (Figure 11). The coupler is only about 8mm across, and serves to link the tuning dial with a variable capacitor which can now be seen in Figure 13.
In a similar fashion, the band selector switch (whose bottom view is shown in Figure 14) is revealed to be a cosmetic makeover for a simple slider switch (Figure 15). Adjacent to the slider, you can make out a portion of the ferrite rod antenna winding for the radio.
The plug for the earpods plugs directly into a hole in the top of the pen (Figure 16). Again, using my penknife, the top of the pen was separated from the body (Figure 17). Looking into the pen body itself (Figure 18), we see the PC board, an LED, variable cap and (before we took it off), the tuning dial. If you look closely on the bottom of the PC board, you can also see part of an electrolytic capacitor crammed in (it had to be bent parallel to the PCB to fit!). There is NO wasted space.
The PC board is fitted into the enclosure by simple friction. After removing the tuning dial and switch covers, it was a simple job of grabbing hold of the PCB with a pair of needle nosed pliers and pulling.
Figures 19 and 20 show the two sides of the PCB. Most of the components are surface mounted, although there are a number of thru-hole parts as well. In Figure 19, you can see a better view of the capacitor first seen in Figure 18. It's a bit worse for wear - a portion of the black exterior was scraped away either during manufacturing or my disassembly. If you look closely on the top end of either photo, you will see two stamped and folded pieces of sheet metal that act as the audio socket. The audio plug fits into the hole shown in Figure 16 and slides between these two pieces of metal to form the electrical circuit. The workmanship in soldering the "socket" onto the PCB isn't the best I've seen, but it is functional.
On the other end of the PCB, you can get a better view of the spring and wire battery contacts first seen in Figure 5.
Figure 21 shows a closeup of the lone IC on the PCB. The part number (CD9088CB) is listed on a number of far-eastern web sites, with no supporting details other than a place to request a quote. Several informal sites on the web mention that this is a far eastern equivalent to the Philips TDA7088T - a complete AM/FM circuit on a single chip. This part is designed to work with voltages from 1.8 to 5V maximum, which is consistent with the two 1.5V coin cells we saw earlier. Supply current is listed on the datasheet as 4.2mA to 6.6mA, with a typical value of 5.2mA. The datasheet for the LR44 coin cells states they have a capacity of 105mAh, suggesting a battery life on the order of about 20 hours.
Figure 22 shows the single discrete transistor on the PCB. Although I didn't trace out the circuit, it's a sure bet that this was to amplify the audio frequency output Signal from the CD9088CB (which doesn't include an amplifier internally) prior to passing it on to the headset.
The controls for that headset are shown in Figure 23. Figures 24 and 25 show the internal structure, which consist of a single 500 ohm thumbwheel potentiometer and slide switch.
If you trace out the PC pattern visible in Figure 25, you get the circuit shown in Figure 26. Interestingly enough, the on/off control has nothing to do with disconnecting the radio from the battery. It simply disconnects the ear pads, one of which is shown in Figure 27.
Figures 28 through 31 illustrate what I found when disassembling the ear pads. At the heart of the affair are two tiny speakers, similar in structure and appearance to their much larger cousins which populate radios and stereos everywhere. Speaker diameter is about 14mm, and the winding and leads are visible in several of the photos. I'd love to learn more about how these tiny marvels are manufactured!
Moving back to the PC board, Figure 32 shows the variable tuning capacitor, LEDs used as AM/FM indicators and the ferrite rod antenna. Figure 33 shows a side view of the same elements.
Photos 34 and 35 show closeups of the tuning capacitor as seen from the side. You can clearly see the capacitor plates. Just like the speakers inside the ear pods, this is miniaturized version of the same basic structure that's been in use for many decades. I was a bit surprised that the internal structure was exposed. Most of the variable caps you will find in distributor catalogs and websites include plastic enclosures. Either the inexpensive & disposable nature of this product, or tight quarters for the PCB, probably account for the exposed structure.
Figure 37 is a composite photo showing details of both sides of the PCB. If you mouse click on the photo, you can view a larger version of the photo. Finally, Figure 38 shows the final result of our reverse engineering project. Again, click on the photo for details.
Tallying up the bill of materials, we've got:
- Two LR44 coin cells
- ear pod headset
- two slider switches
- one thumbwheel potentiometer
- one miniature tuning capacitor
- one ferrite rod antenna
- two small PCBs, packaging & assembly
Conclusion:
It's pretty clear that this device was not targeted to the $2 ASP I paid. A pretty good bargain! If I hadn't cut up the cabling for the ear pods, I could easily have snapped the entire assembly back together and be playing music today.
For more information regarding devices of this type, you might check out:
- Mini FM Receiver using TDA7088
- TDA7000 FM Receiver
- Under the Hood: LifeLong's FM Radio tunes in low-cost analog ASICs
- Poundshop FM Radio
- Coby CX-61 FM Radio Dissection
- Toy radio - pen