Nuts and Volts - November 2013

Re: Cat Alarm, pages 23 and 24, August 2013:

Thanks for the previous help! I've got a few more questions if you don't mind answering them:

1) I think I understand everything in the schematic except for the RF transmitter pins. Why is the out of the LMC555 connected to the ground of the transmitter? Also, what is the input pin used for? I tried looking at the datasheet but it didn’t explain too much.

2) I have started to try and tackle the receiver schematic, as well (Figure 4A in the August column). I looked up the 4013N chip and discovered that it is a flip-flop, and read about what each of the pins do. I still do not understand, however, why the two Q\ output pins are connected in Figure 4A.

3) Also, why are the clock and data inputs connected to ground? I know the clock input is rising edge triggered, but is this merely an alternative form of triggering the trip? For the data input, I am entirely confused what this does.

4) Why are there two resistors (R1 and R2) between the RF receivers out and the 4013N's pins? Couldn't this be accomplished with just one?

Thanks again and if you know of any good resources on these topics for beginners, I would really appreciate it Bill Blackburn

1) The output of the 555 goes high and low because it is connected as an oscillator. When the output is low, it is like a switch to ground which applies power to the transmitter. The transmitter input is connected to VCC, so it will produce maximum output when it is active.

2) The Q and Q/ outputs are connected in order to provide more drive to the transistor Q1. It is possible to do this when the two devices are on the same silicon chip, so they are identical. If you were to try this on separately packaged devices, the switching times would be different and the power dissipation would be increased.

3) The clock and data inputs are not used, so they are connected to ground. It is not wise to leave any input floating because stray electric fields can change the logic level and give unexpected results. I am using the 4013 as a set-reset flip-flop. The output of the receiver sets the Q output high and the reset switch sets the Q/ high. Whatever logic level is on the data input when the clock goes high is transferred to the Q output. The only time I use the data input is when I want a toggle. If you connect the Q/ to D, then every time it is clocked it changes state.

4) You are right; R2 should have connected to the reset line because it is otherwise floating. Mea culpa.

I have not looked into resources for beginners but I am sure readers will have some suggestions;

I will let you know.

Re: ESD Instrument Circuits, page 16, September 2013:

Mr. Kincaid asked about measuring static electricity:

"Two foils will separate when charged due to electric repulsion, but I don't know how to implement that. Does anyone have any ideas?"

I do! Ahem ... I might! What about reflecting a beam of light off one of the leaves which are shiny metal to start with; if I recall properly, displacement of the leaf would create a proportional displacement of the beam. Said beam is hitting — depending on the position of the leaf — somewhere between the edge and the center of a loupe lens. The lens is located over a photoresistor. The light beam ending up striking the photoresistor varies in its area depending on its position, varying the actual resistance of the photoresistor. Appropriate amplification shall be implemented downstream, of course.