Monday, June 30, 2014

Experiment 4 Part 2 (Chapter 4) -- Revisiting 555

If you've read and/or worked your way through Make: Electronics, then you'll probably remember how well the 555 timer chip was covered. That said, it's been a few years since I really dug deep into the chip. I went ahead and re-read Chapter 4 that covers the 555, and I highly recommend it -- you'll be reminded of things about the 555 such as:

* pin numbers on chips start from top-left and go counter-clockwise
* dimple/dot/notch goes at the top
* you can read the pulses by measuring voltage from pin 4 and GND
* negative voltage is always applied to pin 1, positive voltage to pin 8
* 555 is triggered by a drop in voltage on pin 2
* 555 output (pin 3) emits pulse when trigger drops below 1/3 voltage value
* when pin 4 is grounded, output (pin 3) shuts off immediately

To reacquaint myself with the 555 behaviors, I wired up a circuit like the one in Figure 4-3 (page 23) and selected some capacitors and resistors that would give me a somewhat easy to monitor pulse length. Referring back to page 157 of Make: Electronics, there's a chart that provides both resistor values and capacitor values so you can fine tune the pulse length you desire. I chose 10 microfarad capacitor for pin 6 (R1) and 470k resistor for pin 7 (C1) on page 23 for an approximate pulse length of 5.2 seconds. The text tells you to use a 0.1 microfarad capacitor between pin 5 and GND. I put an LED into the mix along with a small On/Off button. To the right is a photo of my setup.

The video will show the circuit in action, but let me tell you what I'm doing in the video that's a bit hard to see. I don't have a simple pushbutton handy, so I'm toggling a SPDT switch on and off with my finger. When it's turned on, the LED lights up. But once I start applying power, I begin counting... 1, 2, 3, 4, 5, 6... somewhere between 1 and 5, I turn off the switch. No matter when in the count I turn off power, the LED will stay lit until somewhere between 5 and 6 seconds. Even a quick flick on and off of the switch will keep the LED lit for the full count. That's the pulse length. My counting isn't perfect, but I am getting a pulse length of around 5 seconds that matches the resistor and capacitor values I picked for R1 and C1. Cool!

Here's the video...and on to Experiment 5.

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