I’ve been tinkering around with breadboards and discrete components in my endeavor to finally get a better grasp of analog electronics. Why do I say finally? Because it’s something that I’ve wanted to learn since I was a kid, and after almost 20 years in embedded hardware development as a PCB designer, software developer, applications engineer, test department manager, software department manager, I’ve never really fully learned. Sure someone can explain a schematic to me, I can grasp it enough to get my job done, but I’ve never been an electronics designer beyond systems and architecture level work.

I find myself now leading a team that is primarily analog electronics based, and instead of making a fool of myself on a daily basis I’ve been putting a lot of time into self study, building circuits, hands on work etc. So back to the bread board, the bench supply, and oscilloscope it was for me. LTSpice has also come in handy for learning, and while it’s quick to build circuits in and play around to see how they work, there’s something more satisfying about building a circuit in the real world instead of simulation.

So this past week I decided to tinker around with the humble NE555 timer for fun. It’s a mainstay of hobbyist electronic projects and not really something used in industry. But it’s a good recap on RC charge and discharge times. First up was a monostable vibrator… *yawn*…. But hey on a recent project at work something like this could have saved our butts (there’s better solutions though). Need to ensure a reset signal needs to stay low for a minimum period no matter how short the reset pulse is? Get a monotable timer set up. A short pulse will trigger the NE555 and the RC timer will ensure that the signal stays in that state for a period defined by the capacitor size and charge current. But yes… boring.

Next up? The astable vibrator. A little more exciting than the monostable vibrator, but really it’s just a fancy word for an oscillator. Not too much fun either. The voltage on the cap used for timing is just fed back into the trigger pin and the device just keeps triggering itself. The charge and discharge resistors set the duty cycle, and there ya go a square wave.

An Astable Vibrator – How Original!

I figured it’d be fun to try and do something else with the oscillator. Why not build a boost converter? It can’t be that hard! A transistor, an inductor, a diode, a capacitor, and a load resistor. The configuration of a boost converter looks something like the picture below.

Oooooo! Fancy! A Simple Boost Converter Attached to an Astable Multivibrator!

Getting a capacitor to charge up with this is pretty easy. First crack at it I managed to charge it up to a whopping 10Vdc from a 5Vdc supply with no load! Ah! but does that mean any real power is being transferred. Adding a 10K resistor to ground from the cap quickly answered that, the supply collapsed below the 5Vdc on the low voltage size. Great! The circuit was bucking in a boost configuration.

A few things I figured out along the way:

My base current was not high enough and the 2N3904 transistor was not turning fully on.

NE555 timers are not CMOS outputs, so their voltage goes down as a function of current, consult your datasheet to see how much! But I was getting an output voltage of 3.5Vdc with a current of 3.5mA of draw. Looking at the datasheet showed that 3.5mA results in a drop in output voltage of 1.5Vdc.

Inductor size and frequency really matter a lot. This was not a surprise and expected.

A few tweaks in frequency here, an inductor change there, multiple resistor changes everywhere, and my piddly boost converter that could only charge a cap to 10Vdc and collapsed with a 10K load was suddenly shooting up the cap voltage fast enough I was concerned about blowing it up. So I put in a 1K load resistor and low and behold it was maintaining an output voltage just under 9Vdc. Success! A whopping 81mW of power at the output of my boost converter built out of an NE555 timer and some components from a project electronics kit I purchased online.

Scope Capture of the Boost Circuit. Yellow – NE555 output; Blue – Voltage measured at transistor side of inductor; Purple – Voltage output of the boost converter with 1K load.

Wow look at that crazy ringing! What is that? Well that’s not good, that’s what it is. It’s the inductor ringing at it’s self oscillating frequency. That means the frequency of the NE555 is too low for the inductor size and is waiting too long to charge up the inductor by pulling more current through it. This kind of behavior will result in a lot of EMI, and general inefficiency in the boost converter circuit.

So how’d it all line up with theory? Actually pretty close! Below is a graph from LTSpice after the circuit stabilized. Note that the purple graph is the current through the inductor, which is not easily measured in the real world. The ringing indeed lines up when the inductor current goes to 0mA.

LTSpice Simulation of the Built Circuit

So there are some differences, the voltage the circuit stabilizes to is higher, though in LTSpice it takes 10s of seconds of simulated time to get to stable, and it does sit closer to the real measurements. The ringing frequency of the inductor is different. This is because the parasitics are not modeled fully. Inductors are probably one of the worst components for effects of components. What’s the saturation current? What’s the resistance of the wire? What’s the inter-wire capacitance? All things that will change how the circuit behaves. Also the LTSpice model of the NE555 acts like the CMOS version not the Bipolar version. It’s output does not drop based on drive current.

This boost converter is not regulated so the output voltage will change significantly based on load. It doesn’t soft start, when first turned on the current draw to first get the 100uF filter cap charged up. All improvements that I might try to add to A Stupid Project – NE555 Boost Converter V2.00 in the future.

So why is this a stupid project? In this day and age you would never go about building a boost converter like this. There are so many small efficient regulated boost converters readily available that are designed for all sorts of applications. Many, along with their support components, will be significantly cheaper than building a circuit this way. But alas, it is fun and a great learning experience to go back to the basics and build something completely from the ground up.