Curious about why my Hz measurement differs between PS and 9v Batt - Page 1
Forum section: electronics / beginners

author: DaMaDo
author_rank: Contributor
timestamp: December 15, 2011, 05:10:12 am
content: I made a circuit to output a 4hz signal for the purpose of bypassing the annoying safety feature that grays out all the buttons on my Toyota Highlander's navigation system.When I hook it up to my cheap temporarypower supplyat either 9v or 12v, I get 60Hz...unless I push down on the 1uF capacitor or sometimes when I touch the bottom of the cap area. Then it goes to the normal 4Hz.But when I hook it up directly to a 9v battery, it goes right to 4Hz perfectly and stays there with no problemsOut of curiosity, what could be causing that?Here are some pics (it's my second time soldering something so the back is not pretty):(Schematic and procedure followed fromJ.A.'s website)

author: IanB
author_rank: Super Contributor
timestamp: December 15, 2011, 05:36:58 am
content: Most likely there is some 60 Hz ripple getting through the power supply and contaminating your circuit. You may need more input filtering, or simply power your circuit off a battery where 60 Hz from the mains won't be a problem.

author: amspire
author_rank: Super Contributor
timestamp: December 15, 2011, 06:30:42 am
content: From the photos, you do not have the meter connected properly.It should be from the output to 0V.As far as I can see, in the battery photo, you only have one of the meter leads connected. In the power supply photos, the black leads looks like it is connected to an open circuit contact on the relay (if it was closed, it would have shorted the meter to zero volts).So in both cases, you are just picking up noise, so it comes down to which noise is bigger - the 60Hz mains or the 4Hz from your oscillator.

author: joelby
author_rank: Frequent Contributor
timestamp: December 15, 2011, 06:49:01 am
content: Do you have an oscilloscope that you can check the output with? One of these would tell you if there was power supply noise present.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: December 15, 2011, 09:40:08 am
content: It really looks like a combination of not measuring to GND and having a 60Hz ripple from the power supply. I'd assume that the ripple is low though and only becomes dominant since you don't measure to GND.

author: Psi
author_rank: Super Contributor
timestamp: December 15, 2011, 09:56:02 am
content: you could put an LED and resistor on the output. See if it's actually flashing at 4hz or 60hz

author: DaMaDo
author_rank: Contributor
timestamp: December 15, 2011, 03:02:54 pm
content: Thanks! Excellent, I guess I still have a lot to learn about ground.I connected the meter to the negative of the PS and it went right to 4 Hz.The reason I didn't have it in the first place was because the website tutorial I used has just the positive connected to the output of the circuit since he's using a battery. I didn't know there would be a difference. Heck, the only reason I had the negative connected in the picture was that I was also making sure I had continuity when the relay switched off.I still don't understand the why of it all, but I still have a lot to read in my electronics books, esp with respect to ground/negative and it's relation to fluctuations.I also connected the output to an LED and it flashes nicely at 4 Hz.This ordeal has shown me points in which I need to concentrate my learning.If Christmas goes as planned I'll have an Agilent U8002A PS and a Rigor DS1102e scope.

author: IanB
author_rank: Super Contributor
timestamp: December 15, 2011, 04:52:54 pm
content: Quote from: DaMaDo on December 15, 2011, 03:02:54 pmThis isn't likely true. "Circuit" means "closed loop", and this is true of electric circuits. Multimeters have two probes because if the electricity flows in, it must have somewhere to flow out again and complete the circuit. You can't make a circuit with only one connection. The creator of the web site must have had two wires connected to make a circuit. Can you post a link to the website?QuoteThis is the most elementary thing. When you first connect a bulb to a battery as a six year old you learn that the bulb will not light unless you connect two wires and form a circuit with both terminals of the battery. Never forget what you already know.

author: DaMaDo
author_rank: Contributor
timestamp: December 15, 2011, 08:06:47 pm
content: Sure,here'sthe websiteAndhere'sa pic of just the positive connected from that website.I guess the negative could be connected to some ground not seen in the picture that has nothing to do with the battery. If this is possible, where can I read about how that works? Having read the basics of electricity so far in my books, they don't seem to cover the whole negative vs external-ground relationship.

author: IanB
author_rank: Super Contributor
timestamp: December 15, 2011, 08:47:42 pm
content: Quote from: DaMaDo on December 15, 2011, 08:06:47 pmYeah, unfortunately the person who posted that picture is an idiot. There are many good resources on the web, but unfortunately many errors and much bad information too. In the picture shown he has the circuit sitting as an island with only the positive meter probe connected. It is working by chance due to capacitive coupling between circuit and probe, but as you found if you touch the circuit the added capacitance of your body can mess up the reading. He needs to have both probes connected in the picture to ensure a positive signal.Quote"Ground" is a human invention that seems to be very common in American circuit diagrams. If you forget the word ground and just think instead of a common return path for currents in the circuit you will be doing fine. The essential elements are that there must be a path, and it must return to the original source of power.There are also protective earth systems (the third wire in mains plugs and sockets), which also called "ground", but this ground is there for a different reason and is not normally supposed to be carrying any current. The over frequent use of the word ground for different purposes and the use of liberally sprinkled ground symbols all over schematics is just done by electrical engineers to confuse people.So to summarize, when you see a ground symbol on a schematic diagram, just think "there must be a circuit, and this marks the return path for the current in the circuit because the designer was too lazy to draw a line back to the origin".

author: DaMaDo
author_rank: Contributor
timestamp: December 16, 2011, 03:51:49 pm
content: Thanks Ian, that helps. Ground seems to be a confusing term for many.