PROJECT: Half-Life 2 AR2, Update #10 - Power Filter v2

Turns out my pi-filter simulation from the previous post does not tell the full story, as with a power filter it's best to first look at the step response before looking at the frequency response. With that said, here are the updated simulations which now include two decoupling capacitors at the load (both ceramic, one 100nF and other 10μF)

Chosen Pi-Filter Configuration

Here is the step and frequency response of the chosen pi-filter configuration. If you want to know how I came to this conclusion then see the following sections

Comparing no filter to pi-filter, note how there is no drastic phase shift and the attenuation is grater, with the -3dB point starting at 70kHz instead of 170kHz

Pi-Filter Step Response

For the step response I am using a current load configured with a pulse function, with the step/pulse looking something like this:

To help with comparison, below is what the step response looks like without any filter present

NOTE: This plot is a bit deceptive as it looks like the circuit is able to respond well to transients, HOWEVER if you also look at the frequency response (see next section) you will see that we can do better

Here is the initial pi-filter configuration I have chosen, a 10μF electrolytic capacitor, a 33Ω (@100MHz) ferrite bead, and a 10μF ceramic capacitor

NOTE: This is not my final configuration, if you have a look at the last section you will see that lowering the ferrite bead impedance drastically helps with step response

And here are some configurations that did not did not make the cut. Again, it's a bit harder to see why if you just look at the step response; if you also see the frequency response (see next section) you will note that some configurations have sharp phase shifts that lead to a "peaky" attenuation

NOTE: I realise that ELEC/CER & CER/CER configurations have a similar step response (with CER/CER actually having a better frequency response). However I am fairly certain that in the real world ELEC/CER will give better performance due to the relatively large ESR of the electrolytic capacitor, as with low ESR capacitors it's much easier to introduce oscillations/ringing with fast transients 

Pi-Filter Frequency Response

Same order as before, first we have no filter (to help with comparison). Note how we have a tolerable attenuation up to 10GHz where he see a drastic phase shift and the peak that comes with it (though I suspect we don't really care what happens after the 1GHz mark)

Here is how things look if we implement the chosen filter from the previous section. Note how there is no drastic phase shift and the attenuation is grater, with the -3dB point starting at 90kHz instead of 170kHz

And for completion, here are the frequency responses of the configurations that did not make the cut. The -3dB point for these (in order) is 230kHz, 170kHz, & 65kHz

Pi-Filter Step Response vs Ferrite Bead Impedance

From here on out I am trying to see what impact the ferrite bead impedance has on the filter performance, the results will shock you ;^)

As before, to make comparison easier here is the step response without any filter present

And here are our candidates. Note how drastic the difference is between the 8Ω (@100MHz) & 120Ω (@100MHz) ferrite beads. As you might guess, am leaning towards the 8Ω ferrite bead for the pi-filter

Pi-Filter Frequency Response vs Ferrite Bead Impedance

Again, to make comparison easier here is the frequency response without any filter present

And here are our candidates, the -3dB point for these (in order) is 70kHz, 90kHz, & 70kHz. Note how the 120Ω ferrite gives +2dB amplification (not attenuation) around 40kHz

Tips For Power Filter Simulation

Here are some things to keep in mind when simulating a power filter:

  1. Make sure to include parasitic elements in your simulation. This would be the parasitic resistance/inductance/capacitance of cables, connectors, tracks, planes, decoupling capacitors, relays...
  2. Include a series resistance in your AC/noise source when doing a frequency response. With my simulation is set it at 10mΩ. If you forget this element (or set it to 0Ω) then your simulation would show amplification before you see the roll-off, while in the real world you would not see a response like this as the source would have a finite series resistance (which I guess is a parasitic resistance)
  3. Know what a typical current transient you are bound to run into looks like for your circuit. For example, if you are switching a relay and you know the could current is 10A then you can better understand what sort of filter response you need
  4. In addition to above, see if you can figure out the constant current consumption of your circuit. Again knowing this will give a more authentic simulation
  5. It's useful to know the input/output impedance of the filter, as with this information you can figure out what filter topology will be best. For example, a pi-filter is designed to "match" a high input impedance with a high output impedance, whereas a T-filter is the opposite, a CL filter is high/low, and a CL filter is low/high


PROJECT: Half-Life 2 AR2, Update #9 - PCBA Block Diagram & Power Filter

Having completed a good portion of the AR2 receiver model I decided to put on my Altium hat and work on the PCBA's. The first board I designed was the RECEIVER_SENSOR-LEFT, which houses the:

  • IR LED emitter pair (used for sensing the solenoid plunger position, see here for explanation)
  • Trigger debouncing circuit (I am using a snap action switch which is notorious for being "bouncy" when changing states)

You can see the location of this board below:

As I was designing the first board I realised that I did not have a solid overview of what other PCBA's will be inside the AR2. Looking back this is something I should have figured out earlier on, especially with a multi-board design like this. And though I had a simple overview posted here, I figured this was not detailed enough...

With all that said here is how each PCBA will be connected, as well as where it will be located:

Power Filter

From the block diagram above you can see that the reciever will house 3 boards:

  • RECEIVER_MAIN-BOARD (primary/mother board)
  • RECEIVER_SENSOR-LEFT (secondary/daughter board shown in video)
  • RECEIVER_SENSOR-RIGHT (secondary/daughter board)

The plan is to generate the power rail (3.3V) on the primary board and then feed it into the secondary board via a ribbon cable, the length of which would be ~30mm. Now, under normal circumstances (as in with a simple project) I would feed in the rail directly as the ribbon cable is relatively "short", but this time I wanted to have a bit of fun and play around with some pi-filter combinations in LTspice. To make the simulation more "realistic" I also accounted for cable, capacitor, & ferrite bead parasitics (resistance, inductance, & capacitance), as these would have an impact on ripple/noise attenuation. For example here is how it looks without any filter present:

NOTE: Below simulations are not ideal for a power filter (should be using step response), see next blog post for new simulations

I should note that with my simulations I am assuming that most of the parasitic elements come from the cable (as in parasitic elements from connector interface and PCB traces/planes are "negligible"). Anyway, after doing a fair bit of research and running many simulations this is the pi-filter combination I narrowed in on:

Here I form a pi-filter with a 10μF electrolytic capacitor, a 33Ω (@100MHz) ferrite bead, and a 10μF ceramic capacitor. The "high ESR" electrolytic capacitor helps to dampen/attenuate any ripple on the power rail; you can achieve similar attenuation by adding an inline resistor but as you would imagine this adds voltage drop to the rail. Also, I went with this configuration as it has a fairly steady phase shift across all frequencies, as in there is no sudden 180° phase shift which would introduce sharp peaks (see below)

Finally, here are some extra combinations that did not make the cut (kinda interesting to see what happens if you only use ceramic or electrolytic capacitors):

And for those still curious, here is what happens when you add a 100nF ceramic capacitor at the output:


PROJECT: Half-Life 2 AR2, Update #8 - Closed-loop Control

In this post I go over my current solenoid/firing pin control methodology and how I aim to improve it

With the current AR2 prototype I am driving the solenoid with a constant frequency/duty pulse, as there is no feedback in the engineering world this would be considered an open-loop system. Doing things this way is easy but as one might expect has some disadvantages, a couple of which are:

  1. The constant frequency/duty pulse must be carefully tuned, and once tuned the solenoid & spring must stay the same if you want reliable operation. For example set the pulse too fast/short and the firing pin would never reach the barrel, set it too slow/long and the solenoid could overheat
  2. As there is no feedback the system is unable to respond to changing/fault conditions. For example, say you are driving the solenoid directly from a lithium battery, well as the battery becomes more depleted the cell voltage will decrease. With an open-loop system this means your solenoid movement will become irregular unless you once again run through tuning

Hence the logical step was to introduce closed-loop control with the solenoid, as doing so would allow me to have the fastest possible firing rate while at the same time make the solenoid movement more resilient to external factors

Open-loop Control

As previously mentioned, this is how I was originally driving the solenoid, as in with no feedback. To go into a bit more detail, the control pulse had a period of ~190ms, ~80ms of which had the solenoid energized. The maximum reliable firing rate I could get in this configuration was ~5.3Hz (in HL2 the AR2 fires at ~9Hz)

Closed-loop Control (IR Photoreflector)

My first attempt at closed-loop control consisted of a couple of IR photoreflectors, one near the start position of the plunger and other near the end. The photoreflectors themselves consist of an IR emitter/LED paired with a matching (wavelength wise) sensor/photodiode, with both being positioned in such a way that the photodiode senses the IR light bounced from a reflective surface (like the polished solenoid plunger):

Results were somewhat promising though not as reliable as I had hoped, as the IR sensor kept getting confused by the non-reflective surface of the spring. Basically giving a fairly irregular firing rate:

Closed-loop Control (IR Photointerrupter)

With my second attempt I repositioned the IR emitter & sensor so that both were in a "line of sight" type of formation (more commonly known as an IR photointerrupter). With this arrangement the solenoid plunger now obstructs the IR light rather than reflect it:

This configuration proved to be way more reliable as the firing rate was now a consistent ~7Hz, which is a fairly good improvement over the original open-loop control ~5.3Hz

Other Options

Of course there are other ways to tackle this problem, for example Airzone has a very good video where he compares Hall effect & IR sensors. With that said here is my short summary of other methodologies:

  1. Using a Hall effect sensor to sense the slightly  magnetic plunger body. One thing to be wary of is that the solenoid will be producing it's own electromagnetic field, so you need to make sure that the Hall effect sensor does not get confused by it's presence (think sensor orientation & sensitivity)
  2. Using an Inductive sensor to sense the ferromagnetic plunger body. TI LDC0851 looks like a nifty sensor though to get best sensitivity it's best to use a planar inductor which increases the design complexity (as you need to use a 4L PCB if you want good accuracy)
  3. Using a Capacitive sensor to sense the metallic plunger body. Again TI has a cool IC, the FDC2214 which interestingly enough can also differentiate through a metallic sheet


RESEARCH: Sunglasses & Attenuation of UV/WHT Light

This project is part of the "why am I jumping to other projects before finishing off my old ones" series...

I have always wanted a compact UV capable flash light and last week got the JETBeam MINI ONE SE (it's surprising how much they managed to cram into this thing...). Well after playing around with the UV light I soon found that I am quite sensitive to it, which would explain why I tend to get headaches after spending a good portion of the day outside

To help with these headaches I have some clip on polarizing sunglasses, and now that I have a UV source I was curious how well they attenuated UV (365nm) & WHT light. So I used my works spectrometer to get some cool spectra plots

Ambient Light

Here is the ambient light spectrum after the spectrometer was zeroed (below is a roughly overlaid humanly visible spectrum)

UV Light (365nm)

0.42W/m² → 0.05W/m² (~89% reduction)

NOTE: 365nm is a bit too close to the spectrometer limits, so there is a bit of clipping happening

WHT Light

75.31W/m² → 22.12W/m² (~71% reduction)


PROJECT: Turnigy Accucell-6 FW Update & Calibration

This project is part of the "why am I jumping to other projects before finishing off my old ones" series...

Many moons ago I tried using my Turnigy Accucell-6 battery charger to log charge/discharge data via DataExplorer, though sadly I could never get it working properly. Well it turns out that the Turnigy Accucell-6 (along with a bunch of other 4 button chargers according to my colleague) is basically a rebranded SkyRC iMax B6 mini, which is fully compatible with DataExplorer :D

So here I attempt to flash the iMax B6 mini FW and calibrate the charger

FW Update (v1.12 → v1.14)

The FW on my charger was changed from:

  • Turnigy Accucell-6 v1.12 (this would have been the original FW charger came with)
  • SkyRC iMax B6 mini v1.14

Interestingly the product page for SkyRC iMax B6 mini shows v1.13 as the latest FW, however only v1.14 has calibration functionality. Also updating the FW is super simple, you just need to connect your charger to the PC (via USB) and run the flash utility "B6mini_SK1.14.exe"


This step is a bit more tricky, luckily SkyRC have a video describing the whole process:

Things you will need to run through calibration:
  1. A charged 6S LiPo battery. Since I did not have one on hand I tried "simulating" one by pumping ~4.2mA into 6 1K resistors in series, but this did not work as the load/resistance (and thus "cell" voltage) changes as soon as you connected it to the charger; meaning you will need to have a voltage source at a minimum. So again my colleague was kind enough to let me borrow his 6S LiPo battery
  2. A calibrated multimeter. I used my trusty EEVblog 121GW which I verified against a calibrated multimeter (Keysight U1241B) at work

From then you basically record the voltage of each cell (remember cell 1 is the one closest to GND) with the multimeter, and then calibrate the charger by going into "BATT METER" and holding "Enter". At this point you enter the voltage (in mV not V) you read with the multimeter for each cell via +/- keys, and once you are happy hold "Enter" again to save & exit the calibration menu

Here is how my charger performed before/after calibration:

Cell # Volt. 121GW, [V] Volt. before CAL, [V] Err. before CAL, [%] Volt. after CAL, [V] Err. after CAL, [%]
Cell 1 4.1865 4.18 +0.16 4.18 +0.16
Cell 2 4.1910 4.19 +0.02 4.19 +0.02
Cell 3 4.1879 4.17 +0.43 4.18 +0.19
Cell 4 4.1909 4.18 +0.26 4.19 +0.02
Cell 5 4.1966 4.19 +0.16 4.19 +0.16
Cell 6 4.1958 4.17 +0.62 4.19 +0.14

So running the charger through calibration reduced the error range from 0.59% to 0.17%, and given this is a hobbyist-tier piece of equipment I am quite happy with the <1% accuracy

A couple of extra points about calibration results:

  1. It looks like the charger ADC/MCU likes to round the voltage down in a curious way. For example:
    • 4191mV will be rounded down (as expected) to 4190mV (4.19V on charger)
    • 4188mV will be rounded down to 4180mV (4.18V on charger, not 4.19V)
    • 4197mV will be rounded down to 4190mV (4.19V on charger, not 4.20V)
  2. If you perform calibration more than once you might notice that the mV reading is not quite the same as the multimeter value. Lot's of other people (see video comments) report the same "issue", which I suspect is totally normal for this hobbyist-tier piece of equipment
  3. I will be mainly using this charger for LiFePO4 batteries which are typically charged to 3.65V ± 0.05V per cell. Knowing that at worst the charger error is +0.19% this means when I charge to 3.65V the actual cell voltage will be more like 3.6569V, which is well within the ± 0.05V limit

Data Logging

As expected DataExplorer now talks without any issues. Here is me charging and discharging a 2S LiFePO4 battery:

NOTE: You can plot fancier looking graphs by exporting the data as .csv and plotting it in your favorite package 


PROJECT: Woodworking & 3D Printing

Sometime in late 2018 my wife and I decided to upgrade her workspace. The initial plan was to tackle this in several sections:

  1. Build the left (self-contained) shelf
  2. Build the right (self-contained) shelf
  3. Bridge the two with a desk

Well we managed to complete left shelf within a couple of months, as for the last two... they basically took us till late 2020. Surprisingly we have COVID to thank for that, as being locked inside with nothing to do gave us a wee boost :D

An interesting aspect I wanted to try with this project is merging woodworking with 3D printing, as I felt the two could complement each other reasonably well especially with smaller fittings. So below I document such 3D printed items:

Small Drawers

Probably the only exception to the "smaller fittings" description. The larger drawer measures 320 x 300 x 70mm and both had to be split into several parts. The only things not 3D printed with these was the metal rods running through the body (for extra support), and the MDF base. Also the body is printed in PETG and fastened with Loctite Control Gel Super Glue

Drawer Handles

Wife wanted something flush yet accessible, so came up with this simplistic design

Drawer Supports

A while back we found a bamboo tray (on the street!) which we decided to convert into a drawer. Initially we were going to mount the drawer slides onto another piece of wood, but after realising how much work would be involved (and how uncool it would look) we decided to 3D print the supports instead

KVM Switch Bracket

Couple of brackets that hold a KVM switch (Unnlink 2 port). Side note, the switch makes it incredibly easy to jump between the laptop & desktop

HDMI Splitter Bracket

Couple of brackets that hold an HDMI splitter (CABLETIME C355). Again super useful for changing between the TV and monitor

Cable Holders

Super simple cable holder. Initially this was twice as tall (so you could cram more cables) but someone wanted a more flush design...

Outlet Bracket

Couple of brackets that hold an 8 way outlet (HPM D105PA8CC). The cylindrical cut-outs act as a cable runway (unfortunately the outlet mains cable was on wrong side...)

Wood Spacer

This one is probably the most "pure" merger of woodworking and 3D printing, a simple 7mm thick spacer that allows the desk and shelf surface to be level 


Fern Pot Brackets

Don't recall the exact plant pot & saucer combo, but suspect it's something like this & this

Laptop Stand (ゴ ゴ ゴ ゴ ゴ ゴ...)

Simple Metabox Alpha N850HJ stand, same one shown here

Desktop USB & HDMI Bracket

Makes USB (2.0 & 3.0) & HDMI ports more accessible, again same one shown here