Custom Circuit Makes for Better Battery Level Display

Isn’t it always the way? There’s a circuit right out of the textbooks, or even a chip designed to do exactly what you want — almost exactly. It’s 80% perfect for your application, and rather than accept that 20%, you decide to start from scratch and design your own solution.

That’s the position [Great Scott!] found himself in with this custom LED battery level indicator. As the video below unfolds we learn that he didn’t start exactly from scratch, though. His first pass was the entirely sensible use of the LM3914 10-LED bar graph driver chip, a device that’s been running VU meters and the like for the better part of four decades. With an internal ladder of comparators and 1-kilohm resistors, the chip lights up the 10 LEDs according to an input voltage relative to an upper and lower limit set by external resistors. Unfortunately, the fixed internal resistors make that a linear scale, which does not match the discharge curve of the battery pack he’s monitoring. So, taking design elements from the LM3914 datasheet, [Great Scott!] rolled his own six-LED display from LM324 quad-op amps. Rather than a fixed resistance for each stage, trimmers let him tweak the curve to match the battery, and now he knows the remaining battery life with greater confidence.

Perhaps the 18650 battery pack [Great Scott!] is building is for the e-bike he has been working on lately. If it is, we’re glad to see that he spot-welded the terminals, unlike a recent e-bike battery pack build that may have some problems down the road.

Continue reading “Custom Circuit Makes for Better Battery Level Display”

Power Wheels Rescued, Restored and Enhanced

It seems power wheels are like LEGO — they’re handed down from generation to generation.  [Nicolas] received his brand-new Peg-Perego Montana power wheels in 1997 as a Christmas present. After sitting in a barn for a decade, and even being involved in a flood, it was time to give it to his godchildren, though not without some restoration and added features. His webpages have a very good write-up, just shy of including schematics, but you’ll find an abbreviated version below.

Continue reading “Power Wheels Rescued, Restored and Enhanced”

A handful of parts used to form an Electrocardiogram


It’s difficult to image a more bare-bones approach to building an ECG. [Raul] used an Arduino nano to collect samples and push them to a computer for graphing.

An Electrocardiogram measures electrical activity around your heart. The white circles above are the sensors which he picked up in a box of fifty for 11 Euros (under $15). Stick them on your skin in just the right places and they’ll report back on what your heart is doing.

He used a AD8221 to amplify the signals. He mentions that this is an ins-amp, not an op-amp. We didn’t find a concise reference explaining what that is. It might be a good topic for the comments section. The signal from that chip feeds into an LM324 op-amp before being dumped into the Arduino.

Simplicity comes at a price. This measures very small electrical impulses and has very little in the way of shielding and filtering. Because of this you may need to do a rain dance, say a prayer, burn a candle, and stick needles into a doll to get a reliable signal on the other end.

Here’s another version that doesn’t require special sensors.

Pulse Oximeter from LM324, LED, and Photodiode

This pulse oximeter is so simple and cheap to build it’s almost criminal. The most obvious way to monitor the output of the sensor is to use an oscilloscope. The poor-man’s stand-in for that is a sound card, which is what [Scott Harden] demonstrates in his write-up.

It uses a concept we’ve seen a few times before. The light from an LED shines through your finger and is measured on the other side by a phototransistor. It’s that light grey plastic thing you see on a patient’s finger when they’re in the hospital. [Scott] went with a common wooden clothes pin as a way to mount and align the sensor with your finger. It is monitored by the simplest of circuits which uses just one chip: an LM324 op-amp. There are three basic stages which he explains well in the video after the jump. The incoming signal is decoupled before being fed to the first amplifier stage. From there it is fed to an adjustable low-pass filter to help eliminate 60Hz noise from AC power in the room. The last stage amplifies the signal again while using another low-pass filter in parallel.

Continue reading “Pulse Oximeter from LM324, LED, and Photodiode”

A cellphone based Interactive Voice Response System

We’re all familiar with IVRS systems that let you access information using a touch-tone telephone. [Achu Wilso] built his own version which uses a cellphone, microcontroller, and computer.

The cellphone is monitored by an LM324 op-amp with an attached 555 timer chip. When a call comes in the voltage on the headphone output goes high, activating the timer circuit. If it goes low and does not go high again for about 25 seconds the call will be ended. Each incoming touch tone acts as a keepalive for the circuit.

An MT8870 DTMF (touch tone) decoder chip monitors the user input. An ATmega8 microcontroller grabs the decoded touch tones from that chip, and pushes them to a PC via USB. The PC-side software is written in Python, using MySQL bindings to access database information. eSpeak, the open source speech synthesizer software is used to read menu and database information back to the caller.

Not a bad little system, we wish there was an audio clip so we could hear it in action.

A VU meter for your PSP

[Christian Doran] wanted some blinky goodness to go along with the tunes on his PSP. He built a VU meter circuit around a couple of LM324 op-amp chips and fit it into the UMD space on the back of the PSP. Using surface mount LEDs and some fine wire he lined up a string of indicator lights round the circle on the clear UMD cover. As you can see in the video after the break, the back of the case now pulses along with the music.

[Christian] notes that building the VU circuit around an LM3915 would have been much easier but he’s working with what he has on hand. Looks like he achieve the effect he was after. If you want to learn a bit more about how the op-amps work, take a look at the tutorial from our links post.

Continue reading “A VU meter for your PSP”

ATmega8 Spectrum Analyzer

[Manekinen] built a very responsive spectrum analyzer. The components at the party are what you’d expect, an ATmega8 does the hard work interpreting data from the LM324 op-amp. This build stands out because it is fast and configurable. In fact, the explanation of the calibration process is where this project shines.

Instead of using water, an HD44780 module displays the spectrum data. The device currently supports several different character displays including 16×2, 20×2, 24×2, and 20×2. We’ve embedded a video of a 20×4 VFD in action after the break. As the video progresses, watch for the Polish words that pop up. This corresponds to the brightness and sensitivity being adjusted with the 5-button keyboard.

Continue reading “ATmega8 Spectrum Analyzer”