555 Timer PWM controlled Servo - Page 1
Forum section: electronics / projects

author: sile
author_rank: Contributor
timestamp: July 08, 2013, 04:11:17 am
content: i'm building one of those most useless boxes ever, where you flip a switch and something comes out and switches it back off again. I could do this very easy with Arduino or TI Launchpad. However, I want to try and build without a micro to make myself learn how to make things without having to write code.Basic operation:1. Everything is off2. Flip switch on, generate a PWM pulse of 2ms with frequency of ~50Hz3. The servo will flip the switch to the off position, this changes PWM pulse to 1ms4. After a 1-2 second pause, time enough for the servo to go back to it's home position, turn self off.This is my first go at it. Based on some wikipedia, some 555 PWM examples, and Dave's simplest soft power switch video I came up with this design.Can anyone think of an easier way to modify the 555 PWM pulse width than method i'm using? Also, I'm not certain how practical it is to use RC circuits as timers when getting down were we need .1 - .2 ms pulse resolution. At what frequency/pulse width do RC circuits typically break down? Also, i'm not sure how the discharge curve on the batteries will affect things. I can simulate successfully with circuitlab, however I don't think it's doing a detailed 555 sim as it still outputs 5V even when I put in 4.4V for the batteries. Then there is temp concern in general as well. In this case temp will be pretty constant.Any other comments or suggestions about how to make this better, more reliable, or use less components?CircuitLabs Schematic:https://www.circuitlab.com/circuit/q27wfs/pwm-servo/

author: Paul Price
author_rank: Super Contributor
timestamp: July 08, 2013, 12:25:03 pm
content: I figured out ten other ways to do it, but why spoil your fun?You have seemed to have found possibly one of the most obtuse ways of accomplishing this task, which I admire!I would have never thought of doing what you are trying to do this way!Hints: You can modify PWM pulse width with:(1) an opticoupler across the 555 timer resistor.(2) Using pin 5 of the 555 timer.(3) using transistors or MOSFET or bjfets to switch in and out timing capacitors for the 555.(4) Using a JFET across one of the timing resistors.(5) Using a bjt across one of the timing resistors.(6) Use a small DC motor to momentarily turn on and adjust the timing resistor pot it is mechanically coupled to. You can easily salvage this kind of motor from a discarded CD-ROM reader, the tiny motor that opens and shuts the tray and it comes with a nice drive belt to couple to a potentiometer shaft.(7) Just use the same motor to reset the switch and turn itself off and skip steps 1-7.Hint:I guess I am a little old fashioned, but I am always leaning towards creating state-machines to do and keep track of sequential operations in a easy way, even if it means I shun the use of a MCU and use one or two discrete digital CMOS circuits to count clocks to establish timing and remember the progress of a control sequence.That's why the almighty invented flip-flops and shift-registers and counters and MCU's to count clocks.After all, you are using the 555 as a timer, and you are counting clocks(period or frequency) in some way, anyway.You can sequence 555 timers as well where one turns on the other until one turns them all off, but even using diodes to channel or steer the progress of this sequence is a resort to digital circuit logic and you might wonder how simple it is to do things that way.

author: Tepe
author_rank: Frequent Contributor
timestamp: July 08, 2013, 12:36:35 pm
content: Quote from: sile on July 08, 2013, 04:11:17 amYou do not need any electronics at all. A couple of suitably selected switches will suffice.