Gregory's Serveri was looking at the Wikipedia list of 7400 chips (https://en.wikipedia.org/wiki/List_of_7400-series_integrated_circuits) and noticed some really unusual devices.🧵
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the first device is easy enough to find, but still odd: the 74HCU04. the "U" stands for "unbuffered", meaning that it is literally a 2-transistor inverter. a regular 74HC04 has several stages of CMOS inverters connected together. to demonstrate the chip, i'm driving the input with a sawtooth ramp instead of a digital square wave. this lets me extract the VTC (voltage transfer curve). here you can see that the transition is quite soft; in-between voltages that are neither logic 1 or 0. putting the scope in XY mode gives you the actual VTC graph of output versus input. the X axis is the input voltage and the Y axis is the output voltage. the bottom left corner is 0,0. for contrast, this is the VTC of a standard 74HC04 (with buffering). the multiple stages of transistor inverters inside the chip sharpens up the transition region. 74HCU04 devices can be used to build oscillators and even amplifiers! this snippet of the Snappy Video Snapshot circuit shows how they used a similar chip as an inverting op-amp to build an active low-pass filter! next up is the 74LS362. it is a clock generator. you connect a crystal or square wave source and it produces four non-overlapping phased clock outputs. there is a separate VDD rail that can handle 12V for level shifting the outputs. i think this part was used in the TI 99/4A. this is the 74LS624, which is a VCO (voltage controlled oscillator). i have no idea what it is doing in the 74xx series since it is totally analog!   @tubetime This is so interesting. Alot of guitar players use tube preamps because they like the sound. I wonder if this chip could simulate the transfer curve of a tube preamp. That would blow me away: seeing a digital chip as the input to a nice-sounding guitar preamp! @beeftacos a tube preamp's VTC actually folds back on itself. this creates 2nd order harmonic distortion. fascinating stuff. @tubetime Seriously? I didn't know that. Maybe that's where the unique electric guitar distortion sound originally came from? @tubetime There's a monosynth called the Wasp that uses similar CD4069UB inverters in their linear range as amplifiers and distortion effects.  |
see, over the past few days i've been sorting through a *ton* of chips that i recently got. and, as it turns out, i actually *have* several of the truly strange devices in the 74xx family. so let's take a look!