I Salvaged a Useful IC From an Old CRT Monitor

Added: 2026-06-01

[00:00:00]Recently, I needed a circuit that makes a sound like this.

[00:00:07]Scientifically speaking, it responds to a drop in the input signal, also known as a negative edge, >> [music] >> and outputs a brief audio signal of a fixed duration.

[00:00:18]Try to guess what I need this for, and I'll tell you a bit later.

[00:00:22]Of course, you could spend a couple of bucks and build this using a microcontroller, assuming you know how to write code and flash [music] a chip.

[00:00:32]But I wanted to build it completely computer-free, using just transistors or discrete logic gates.

[00:00:39]At first, I planned to build the circuit entirely out of discrete transistors.

[00:00:44]But in my last video, I showed you what kind of cool stuff you can salvage from old electronic junk. And among the junk was a board from an old CRT computer monitor.

[00:00:56]I took a closer look at that board and found this exact chip.

[00:01:03]>> [music] >> The HEF4538B integrated circuit consists of two identical blocks, each known as a monostable multivibrator.

[00:01:14]>> [music] >> In that monitor, it most likely generated vertical or horizontal sync pulse sequences.

[00:01:23]The core of my problem is that I need to catch the exact moment a voltage drops from a high level to a low level. Then the output needs to trigger a fixed-length pulse, which will create that exact blinking or beeping sound.

[00:01:38]A monostable multivibrator does exactly that. It generates that precise pulse.

[00:01:44]Just what the doctor ordered.

[00:01:47]Each of these modules has two inputs.

[00:01:50]One responds to a positive edge, and the other to a negative edge. [music] Both of them set an internal flip-flop, which automatically resets itself after a certain period of time.

[00:02:02]You can also reset the flip-flop manually via dedicated reset input.

[00:02:07]Additionally, each module features two outputs, a direct and an inverted output.

[00:02:14]So, it's a pretty versatile block for just about any scenario.

[00:02:20]The time it takes for the flip-flop to reset >> [music] >> depends on the resistance and capacitance of the resistor and capacitor connected to these dedicated pins.

[00:02:30]The audio pulse duration I need is roughly between 100 and 200 milliseconds.

[00:02:37]The data sheet provides a formula to calculate the reset time, but it doesn't specify the units of measurement. Let's try to figure it out experimentally.

[00:02:47]>> [music] >> I'll desolder the chip from the board and pop it into an adapter.

[00:02:56]I've connected different colored LEDs to both the direct and inverted outputs.

[00:03:02]So, we can visually track the state of the flip-flop.

[00:03:06]All right. I've got both halves of the chip fired up.

[00:03:11]Each pair of outputs go to this own set of LEDs, and the inputs are controlled by just two buttons. Each button sends a signal to both halves simultaneously.

[00:03:22]However, their RC circuits are different. The capacitance over here is five times higher than the over here, but the resistance for both is the same, 100 kiloohms.

[00:03:35]By the way, I figured out the units of measurement. Turns out I just completely missed it.

[00:03:42]They're listed here in the examples, but they're actually messed up, too, right here. [music] The circuit runs on 3.3 volts. When powered on, it immediately fires a pulse.

[00:03:56]On a side note, this chip was picking up every bit of interference in the universe. From my finger, the multimeter, the weather on Mars, you name it, until I tied the ground to the negative power rail. After that, everything started working perfectly smoothly.

[00:04:15]So, the left input on the schematic is pulled down to ground through a resistor. And pressing the button connects it to the positive rail.

[00:04:23]Meaning when [music] pressed, we get a positive signal edge.

[00:04:27]The right button does the opposite.

[00:04:30]Pressing it creates a negative signal edge.

[00:04:33]Both actions trigger the flip-flop in exactly the same way.

[00:04:37]The top half delivers a longer pulse duration because of the larger capacitor.

[00:04:42]The pulse duration stays exactly the same, no matter how long you hold the button down.

[00:04:49]This is true whether we release the button before the output pulse finished, or we hold it down for a long time.

[00:05:05]I connected the LEDs directly to the chip's output through resistors.

[00:05:10]The current draw for the LEDs here is only a few milliamps, which is well within the chip's specs.

[00:05:17]However, we still can't get away without transistors. [music] For the audio signal, I'll be using this module right here.

[00:05:26]It puts out a very loud sound at a frequency slightly over 2 kilohertz.

[00:05:32]When supplied with a DC voltage anywhere from 3 to 7 volts.

[00:05:37]The maximum output current of this chip is only 10 milliamps, but this buzzer draws 15 milliamps.

[00:05:44]That's why I added a simple transistor switch.

[00:05:47]>> [music] >> It will provide enough current to drive the speaker.

[00:05:58]>> [cheering] >> All right, the buzzer is working. And for what I actually needed it for, I'll show you in very next video.

[00:06:11]So, make sure to hit the subscribe button if you haven't already.

[00:06:15]And I'll be looking out for your guesses in the comment below. It's going to be a fun one.

[00:06:23]See you next time, [music] and may your ideas come to life.