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A while ago I built a simple constant current dummy load for testing my SMPS however the maximum load was about 1 amp and 2 amps with a fan on it and I always had to use a multimeter to measure the current. I’m looking at having a load of up to 4 amps or more, adding an ATtiny to control a 4 digit LED display to show the current, use a proper potentiometer and use a bigger heatsink.

Originally I was planning to have it all powered by input voltage like before however at low voltages the STP22NF03L N mosfet that I had lying around didn’t switch on enough with the MCP6242 op-amp so after thinking about how I could do it – use 2x coin cell batteries, 9V battery, etc, I went with a Lipo battery. An upside of using a battery is that this allows us to use any input voltage between 0V to 30V to test our load with.


Before I had 1 ohm resistors which were almost getting as hot as the mosfet so I’ve gone down to 0.1 ohm 3W resistors – I added the option of another resistor in parallel if need be. I used a 1.5K resistor on the op-amp on the non-inverting side of the op-amp for a bit more fine tuning of the pot.


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Last year, I dismantled my 15 year old RC car and re-built it with an AVR, nRF wireless and N mosfets for the H bridge.


It was all working well until I decided to charge up the Lipo batteries to full which was just enough to voltage to fry (and catch fire) one of my high side mosfets from the voltage doubler (should have used boost strapping instead). Now it’s time for me to re-visit this project – use P & N mosfets, change the wireless to 433MHz for more range and add some controllable front/rear lights.

Originally I was going to use the ATtiny85 but since I want the lights to be controllable I need a few more pins so I’ll be using the ATtiny84. Most of the 433MHz receive/transmit code can be re-used from my Alarm system – Remote control upgrade and most of the rest could remain the same.

I decided to simplify the PWM to the motors and have it done all in hardware rather than using timers with interrupts like I did before.


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From our last post we revisited the MC34063 circuit to see if improvements could be made in which we found reducing the turn off time helped quite a bit and we also made a constant current dummy load for testing. Also there was a user who left a comment about using a NPN, diode and resistor to improve switch off time, it reduced it by more than half and I saw 40C on the FDC365P mosfet. The MC34063 circuit has become a bit much as it stands in terms of the components/size so I went looking around for another DC-DC chip which costs around the same and is more modern.


I found the Richtek RT8293A on special for 40c which is step-down converter, 4.5V to 23V input, 0.8V to 20V output, 340KHz operation and up to 3 amp output. There is a heatpad on the bottom of the SO-8 package for better heat dissipation.


We adjust the output voltage by the resistor divider like before and since we are at a higher frequency we can use a small inductor though there are a few extra components to the RT8293A circuit too but they only add a few cents more to the cost. There is the 3.3nF and 13K for the error amplifier compensation, a soft start feature which uses a 0.1uF capacitor to slowly increase the output voltage, another 0.1uF capacitor to bootstrap the high side driver, a 100K for the chip enable and they recommend 2x 10uF input and 2x 22uF output ceramic capacitors.

PCB Test


So I went ahead and bought everything  that the datasheet said and started designing the PCB around the layout that they provided but looking around there is a reference board made more recently and the layout made a bit more sense so went with that.


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Today we’ll be taking a look at the Technicolor TG797n v3 Router (Telstra branded), an ADSL2+ Wireless router with 1 gigabit WAN and LAN port, 3 10/100 ports, VOIP, DECT and 2 USB ports. Once again this is another router which I haven’t heard of this brand before. It looks to be similar to the Thomson Gateway TG797.

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2 screws later and we’re in. There’s a small removable panel at the front which looks to be for a DECT cradle.


We’ve got a MP9141ES and 2x MP1492 SMPS near the input jack, an UTC LD1117 LDO and we’ve got Lelon branded electrolytic capacitors, there is a outline in the middle of the PCB for a very large capacitor. The PCB date code is 14th week of 2014 so it’s pretty recent. The heatsink that’s on top of the main chip has been soldered down using what looks like a nail on the top left and some epoxy to hold it down, the heatsink has a foot print which could have been a bit bigger. There are two metal cans, one for the DECT radio and one for something else labelled SC – which there are two more labels on the PCB, I can only assume it means “Special Circuit” or similar.

Looks like we’re missing the transformer from the PSTN black port but it’s still active and the green port is inactive according to the manual. There are 3 different transformers for the Ethernet ports, 1 that covers 2 gigabit ports, 1 for 1 LAN port and another one for 2 LAN ports.


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From our previous post, the cases/mounts were built, a few more smaller changes were made and we added in the SMS capability. I thought that was the last change I would do to the alarm system for a long time but it turns out that the range of the remotes using the nRF24L01+ wasn’t very good, when outside the house you had to be within a few metres. This time we’ll be looking at adding 433MHz wireless to our system for the remotes to improve our range.

(sneak peak)

When I found the remotes range wasn’t great, I tried switching to 250Kbps operation of the nRF however I found that firstly there was an issue with the PCB antenna, sometimes it would work fine but other times I had to touch the antenna and then it works.


I added in a extra bit of wire to the antenna and that sorted it out though the range didn’t really improve much and by that time I had already switched the door/PIR sensors to 250Kbps too so I was stuck with it. So I needed a solution – could I possibly re-wire the old alarm system remotes to transmit the data I want?



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Previously I made a small solar power garden light with an RGB LED controlled by an ATtiny13 and I have been thinking about how I could create a different project from it. The ability to control these RGB LEDs via wireless on demand seemed like an interesting idea – potentially you could program a sequence and have them execute it to give you a small light show if you had enough of them.

(sneak peak)

After browsing for an AVR, I found the ATtiny841 that has 3 timers which is exactly what I need to control each LED properly with PWM (another solution would be to use a WS2812 LED).


Each ATtiny841 was $1.3 so you can’t really go wrong, I didn’t know these MCUs were available so it’s a good idea to regularly browse supplier’s websites for new products. For the server side, I’m thinking an ATmega with 16×2 LCD and keypad to enter the sequence.

Client side


The ATtiny841 gives us the ability of selectable output pins for the timers which you’ll need to configure to have any output from the timer at all plus you also need to enable the timer output enable of the pin too.

rgb5 rgb6


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Today we’ll be taking a look at the EnGenius ESR9850 Wireless N Gigabit Router, a wireless router with 4 gigabit ports, haven’t heard of this brand before.

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4 screws later and we’re in.

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PCB layout looks quite good, we’ve got a ceramic heatsink on the main processor, 2 RF outputs without the metal cans with the antenna cable soldered on, 2x CAT7125CA SMPS up the top right plus a smaller U8AM878 one near the bottom center and a TX/RX unpopulated connector and an 8 pin unpopulated connector – likely JTAG. PCB date code is 2nd week of 2011.


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From our last part we moved over to the ATtiny24, our PCB arrived, tested good and made a quick case for it. This time we’ll check out the expansion board, a new case with changes needed to make the case work and the KVM board is now available for purchase.


The expansion board has arrived, it’s basically the same as the main board except it’s got a input expansion port instead of the VGA port. One thing I forgot to take out was the USB ports for the expansion board, so potentially if you didn’t want to use the main board’s USB connectors you could use them (I’ll take them out of the next version).


I bought some 20mm buttons and was able to solder in a 2 pin female header on the LED SMD pads so I could attach a 3mm LED to it and added some glue to hold it all in place.

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After changing the case colour to black it looks nicer and after connecting the expansion board with case too, it fits pretty well and it works. I need to put some labeling on it, the only viable method I’ve found so far was to make my own stencil from acrylic and then spray paint the name on it, you have to do a light spray otherwise it’ll seek through the stencil.


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Today we’ll be taking a look at the Power Shield PSO-650 650VA Powerboard UPS, a 3+3 output UPS with surge/battery protection, phone line protection and USB for monitoring it on your PC. This particular unit failed because the 12V battery dropped to around 2 volts.

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By looking at the power cord is going to tell us that this UPS is going to be built down to a price because the cord comes straight out of the UPS itself.


We’ve got our main PCB on the bottom and a huge transformer on the top which isn’t glued or bolted down, it just sits there due to the pressure the case puts on either end when closed. We have a 250V 5A circuit breaker on one side of the case. The surge protected outputs have some big MOVs on them. There are a few things which could be a little better, the heat shrink may have been too big that it’s pretty easy to move around and they left in an extra wire like it was meant to go somewhere else (not that it matter too much as the terminals screws are exposed anyway). The transformer is a 1065SP-2-V8/ E186x40.


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We recently purchased a Toshiba Satellite Pro L50 for a client, took it out of the box and started setting it up. About 2 hours later, I hear a bang and the circuit breaker trips. After smelling the power adapter; it smelt bad so it was likely blown, it was sitting about 1 metre away from me. Luckily the laptop survived and to Toshiba’s credit they sent a power adapter which arrived the next day (I left it running for 24 hours without issues).


The power adapter is a 19V 3.42 (model PA5178E-1AC3, part PA-1650-60).

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After taking a dremel to the plastic casing, we’re in. The metal shielding is connected to earth via a 1K resistor. So far no signs of damage.

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IMG_3874Now we see lots of charring on the bottom of the board and most of it is pointing to around the front where the mains is connected near the fuse. Just above the black connector we have the diode rectifier, notice the resistor chain to the left and the PCB’s silkscreen has good markings where the pads, rubber or glue should go.

After desoldering the fuse, we can see that it’s blown pretty badly, it’s a 250V 3.15A. There must be some reason it’s blown or not?


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