Not so much a circuit I build this week, but rather a commercially bought circuit/shield that I thought I'd have a crack at repairing. I ordered some 18650 battery chargers which also double as a power supply (claimed 3V-1A and 5V-2A output). Everything looked wonderful until I didn't read the many warnings on both the board and the website and stuck an 18650 battery into the holder with the wrong polarity. ZAP!! Burnt finger for me, and magic smoke production for the charger.
Always read the fine print
I suppose a normal person would throw the whole thing in the bin and look for a commercial replacement at many times the cost. But after taking a closer look at the broken board I thought that only one component looked completely spooked by the experience - a little 8-pin TSSOP package sitting on the edge of the board. I couldn't tell what the label was through the charred remains, but in an un-fried version of the same board it seemed to be a FS8205A Mosfet Battery Protection IC. The cost of a new charger/power supply is around AUD 3.81, and cost of 10pcs FS8205A is around AUD 0.77, so no problem - order and wait. When the chips arrived, it was simply a matter of de-soldering the fried chip using a hot air gun, and then plonking on the replacement and re-soldering. I do enjoy the SMD soldering and if you are a little wary of it then order one of the many practise kits available and have a go - it's not that difficult. A pair of comically large enlargement glasses can help as well.
Normal operation
Fried guy
De-soldered old chip with keen replacements in the background
Re-soldered and ready for action
The end result is a working charger and power supply, and a glow of satisfaction for it's owner. Nice one.
Postscript: the "un-fried" charger went "fttt!" as well with the same component at fault, and this time I didn't put the battery in the wrong way, so obviously a bad batch of mosfets and probably the reason for offloading stock cheaply to unsuspecting Tasmanians half a world away. Anyway, a desoldering and resoldering session ensued and now I have two perfectly fine chargers/power supplies!
So the lovely 555 based square wave signal generator PCB arrived from JLCPCB. The story so far:
Dream up a suitable project to illustrate how easy it is to make a PCB, this one based on the 555 timer which can output a square wave signal of variable frequency
Use online software to connect all components in the circuit in preparation for ordering and manufacturing
Layout PCB design and order the PCB, then wait by the postbox for a couple of weeks
So now the only thing left to do is to assemble and test the PCB. Note that I used the TLC555 version of the 555 timer which is CMOS based and not only uses a lower voltage, but also requires very little current in operation. The TLC555 can also output a much higher frequency than the bog standard NE555 - 2MHz vs 500kHz. There are other versions of the 555 around as well - nice chip!
Some voltage regulators are known for their inefficiency, and some for their stability. The HT78XX series of regulators are touted as:
... a series of positive, linear regulators feature low quiescent current (5uA typ.) with low dropout voltage, making them ideal for battery applications. The devices are capable of supplying 500mA of output current continuously. They are available with several fixed output voltages ranging from 1.8V to 5.0V. Although designed primarily as fixed voltage regulators, these devices can be used with external components to obtain variable voltages and currents. These rugged devices have Thermal Shutdown and Current Limiting to prevent device failure under the "worst" of operating conditions.
I wondered if "500mA of output current continuously" is true, and also what are "worst" conditions and can I replicate that? Firstly though, what do you do with SOT-89 packages if you want to test a component on a breadboard in a circuit? I found a few options in the buckets including a legit SOT-89 to DIP adapter, a SOIC-8 to DIP-8 adapter and finally a standard two sided blank prototyping PCB.
These little blighters aren't cheap, and one thing spotted AFTER ordering (¯\_(ツ)_/¯) is that they have a max input voltage of 8V - not much compared to say the LM78XX which is happy with a 35V input. Anyway the "fun part" for me is the soldering so away to the bench.
Clipped prior to soldering
On the way to making the adapter
The DIP-8 version having headers added
The frankenstein version on a bog standard PCB
So now that we have the soldered versions ready to test - let's try a few variations on a theme, using the suggested "basic circuit" from the datasheet with a load of a resistor and 3W LED.
The last thing to try is maybe use this little linear regulator instead of a zener diode in the QX5252 circuit.
So you can see from the above that at 1V input to the joule thief, I replaced the 5.1V Zener with the HT7805 and it works! Not only for an LED, but wafting the ergs out to an Attiny13 with a blinky sketch also worked fine - now that could be very interesting for the future - watch this space.
Take two on this chip as those in the bucket originally were FAKE! So I got my money back from AliExpress and re-ordered. The wait was worth it because they turned up a few weeks later and actually worked as advertised - nice. There was only one unanticipated problem - the outputs from the CD4026 chip are "1" for "HIGH" which turns on each segment from a common cathode 7-segment display, but sadly if you only happen to have common anode 7-segment displays in the parts bucket then the result takes a little interpretation.
Red = what is lit, Green = what should be lit!
I have ordered some common cathode displays to make better (easier) use of the chip, but in the meantime I looked for a solution - is it possible to turn all "1" signals to "0" and vice-versa? Well, yes! You just need an IC that has multiple NOT gates - for instance a CD4069 Hex Inverter as per last week's blog would be perfect, just add one transistor based NOT gate to complete the seven required inverters and hey presto we have a display that works on "1"s and not "0"s.
In "real life" the result is as shown in the video below.
Eagle-eyed viewers will spot a lone resistor going from the transistor not gate to segment D. There's a case to be made for resistors on all outputs as seen in many circuits shown online. I just threw one in here as the single segment output from the transistor not gate was showing up brighter on the display and it became a little distracting. A 220Ω resistor cured the imbalance and the resultant overall display was more evenly lit.
