We've got a mouse in the house. I want to setup traps in the attic, but I don't want to go up there to check them. My thought is to use a simple circuit that uses the trap itself as a switch. I think it needs two circuits: a path for the electricity when the trap is set (circuit A) and a path when the trap has gone off (circuit B).   I think:   Circuit A should be nothing except a resistor. (If it was just metal, the battery would run down real quick... plus... you know... hot). Circuit B should have a resistor and an LED (and a long pair of wires so we can hang the LED out the attic door and it is visible from the hallway.   When the trap is set, circuit A will have less resistance than circuit B, and thus short circuit the light. When the trap goes off, circuit A will break, and B will then be the only path for the electricity thus lighting up the LED.  Questions: How do I calculate out what resistors to use?  In terms of power, how do I decide on 9V? 5V? 1.5V?
33467
I have a simple circuit:    The max rating (current) of the LED is 30mA.  How can I work out what the resistance of the resistor needs to be? By using ohm's law I found it to be \$3V/0.03A = 100 \Omega\$. However using software called , and trial and error I got the minimum possible resistance to be 36&Omega;. However, if I use a 35&Omega; resistor, then the LED breaks. Is the software wrong, or (more likely) am I doing something wrong?
41841
Consider two circuits containing a battery, bulb, switch and conducting wires but of length 1 meter in one case and 1000KM in other. When switched on at the same time both the bulbs glow instantaneously. I think it is because, electric filed travels from positive terminal of battery to negative terminal through wire almost instantaneously.  My question is there any speed for this electric field? If yes, what is it? By any modification in this circuit is it possible to induce any delay in glowing the bulb?
71022
While replacing a light switch, I shut off the circuit breaker for it and checked the voltage to be sure it was not hot. I noticed that the hot wire read 10VAC, which was odd. It was pretty consistent. Just on a hunch, I went and shut off the circuit breaker for another circuit nearby (the GFCI circuit for the bathroom outlets) and the 10 VAC went away.  I checked the neutral wire resistance to ground, and it was very low, which is correct.   Any ideas on what to look at for this?
118443
I was just wondering what happens in a circuit in terms of different types of energy transformations. If you apply a voltage to a circuit then electrons start moving (very slowly). Since the electrons want to flow to the positive terminal they will have electrical potential. But the electrons are also now moving once the voltage is applied as there is a current so would they have kinetic energy as well?   Some information I have found online says that electrons collide with atoms in a bulb and this is why the filament heats up. But this would imply that kinetic energy is being transformed into heat and this can't be correct because surely any change in kinetic energy would alter the current flowing? Also, in a series circuit, if you measure the potential difference between any 2 points after all of the loads then you always get zero. I was just wondering why it is zero because don't the electrons keep moving even after passing through all the load in order to get back to the power source so surely they cannot do this without some form of energy?
41155
I have a simple circuit that runs off of a 9V battery. I'm re-designing it so that it can also run off of an external 12V DC source (ie: a wall adapter). I want to design the circuit so that if both the battery and the wall adapter are connected simultaneously, the wall adapter is used, and the battery is effectively disconnected from the circuit.   I've found a , but they , and since it could be a non-rechargeable (ie: alkaline) cell, this could be disastrous.   I've considered the using a , but I'm not quite sure how to start. How would I go about designing such a circuit?
88303
I'm a Electrical Engineering student and I want to sense and sample the voltage signal  coming from a wall socket (110V - 220V). I came up with the following circuit using a voltage divider with high impedance and a diferential amplifier. I would like to avoid using a transformer because of weight constraints.  Is there a better way to solve this problem? Should I use a capacitor divider instead?
30238
Note: I am very new to the more advanced side of electricity. I do not have a firm grasp on the exact mechanics.  Lets say I have an example circuit:  5V Power Supply -&gt; LED -&gt; Resistor -&gt; Ground  Now the LED is receiving 20ma and 5V. If I were to switch the LED and the resistor however, the LED should receive less voltage but the same current, so wouldn't it be dimmer? Or would it receive the same voltage?  I've looked at these two threads and their answer doesn't help.
19047
I am taking my first steps into learning about electronics as a hobbyist and I am struggling to understand something about resistance.  I have read about Ohm's law and it seems fairly straightforward so I set up a simple circuit on .    This circuit consists of a 9V battery and a single resistor (10 Ohms) with its leads connect directly to the 9V battery.          &ndash; Schematic created using   When I place the probes of their simulated voltmeter across the leads of the resistor, it reads 7.83V.  I can't make any permutation of Ohm's law produce that result.  What would be the expected voltage across the resistor?  Based on other posts on this site, I would expect the voltage to be 9V.  I don't know how that simulator (123D) calculates its values or how it simulates the circuit.  I am sure that I am missing something fundamental but so far I haven't figured out what.  Is the 123D circuit simulator right?  Should there be only 7.83V across that resistor?  I set up a similar circuit (using CircuitLab) and it seemed to show that the voltage would be 9V.  So I am confused.  Is this just an inaccuracy of the 123D's simulator?  Can someone help me to clear the fog?
9516
I would like to know how can i add a safety switch with an LED indicator to a circuit , so that if the safety switch is on then the LED lights , and when i want to use the device , all what i have to do is to press the normally-open switch.   I have attached this schematic and im not sure if it will work correctly  ,as i am a very beginner in electronics   Thanks
15576
Answers to other questions here (e.g. ) show capacitors used to drop AC voltages to levels suitable for a LED. I've seen much simpler circuits such as the one below         &ndash; Schematic created using   Assuming it's in a suitable enclosure, are there significant drawbacks or dangers to this circuit compared to others?
10886
When I was young and learning about electricity, a fabulous tool for understanding voltage/current/resistance was an incandescent lightbulb (in my case it was a small 3V bulb). When you doubled the voltage by putting two batteries in series, it glowed 4x as bright, but heated up more and was more prone to burning out. When you put two lightbulbs in series, they glowed 1/4 as bright. When you put them in parallel, they glowed normally, but drained the battery twice as fast. Etc.  This day and age however incandescent bulbs are on the way out, and LEDs are replacing them for a good reason (like not burning out every few months or so). But LEDs are different and follow different rules, which I don't understand myself very well.  I was wondering - can LEDs be used in the same way? I know that for a LED to be usable in a similar way as a classical lightbulb, you need to put it in series with a resistor, otherwise it draws too much current and burns out. I think you can even buy LEDs with built-in resistors. But would they work in the same fashion? Would changes in voltage be accompanied with corresponding changes in brightness?
8088
In my quest to understand electrical engineering, I have stumbled across this tutorial:    I have understood the diagrams until I got to switches. I am not sure how switches work on the breadboard or the diagrams. This is the specific one I am thinking of (this is of a pull down resistor):    The implementation is:    Based on the diagram, what I think is happening is: Power goes to the switch, if the button is up then the circuit is not completed. If the button is pressed then the current takes the path of least resistance to pin2 because it has more pull (100ohm &lt; 10kohm).  The way it is described in the tutorial sounds like when the button is up, the circuit is still complete, but the 10k ohm resistor pulls the power to the ground. I am not positive how or why if both the 10k ohm and 100ohm are receiving equal current, the current would get pulled to the ground through a higher resistance than is open to pin 2.
84139
I am a beginner with LED lights, but would like to use a 5mm UV LED to make a pendant with a small fluorescent crystal. (This is what I purchased ()). I would like to create a very simple, very compact circuit with a coin cell (watch battery), and an on/off switch. What would be the easiest way to do this? Also, would using 2 batteries fix the voltage issue?
118373
What’s the worst that can happen if i put a switch in the neutral instead of the phase?   I just found out that 3 light bulbs sockets in my home  are connected this way, it first came to my mind to fix it but then I recalled the flow of current from positive to negative is just a convention since the electrons are charged negatively and they actually flow from N to L so i cant really find an solid excuse to mess with the wiring for the next 2 hrs
74403
If I had a 12V battery and wanted to use an LED that used 2V and 20mA, then I need a resistor that will drop 10V and provide enough resistance to produce a current of 20mA. Therefore I would use R=V/I = R = 10/0.020 = 500 Ohms.  Therefore I would need a 500 Ohm resistor. I believe this is correct so far?  My confusion comes from when we then look at the circuit as a whole. The circuit has a current of 0.020A and a resistance of 500 Ohms, which when looking at V=IR equals 10V (the resistor), not the 12V that it should be there for the whole circuit?  Where am I going wrong? How do I make V = IR work regarding the whole circuit so that V = 12?  I'm pretty new to this sort of thing and no doubt I've probably made a daft mistake but if someone could just help clear up my confusion it would be appreciated.  Thanks.
62775
Suppose we have the "basic" stuff like a battery 2 piece of wire and a bulb. Battery has a potential difference. But from where electrons flow to make the bulb light? from wire or from battery or from both? also if electrons flow from battery and they go through the wire (conductor) then why in insulators this doesnt happen? insulators dont give electrons but why they dont let electrons flow?
41692
Long time reader, first time poster. Working on updates for our new house and I bought a few lutron caseta switches the other day to wire up in our front entryway. I went with the PD-6ans which requires a neutral wire. I pulled the plate off and looked around and saw a neutral bunch so I thought I was good to go.   Well, like any project I get myself into, things didn't go as planned. The two switches on the left come from one circuit with the neutral bundle, the switch on the right is on a different circuit and there's no neutral.  Can I use the neutral bundle from the other circuit in the box or is that a big no-no? If not, is there any other way to get around it?  I'm thinking I may just need to buy a different switch that doesn't require a neutral (Lutron Caseta PD-5WS).  Here are a few pictures of the three gang box. It's a three way switch on the far right and left, single pole in the middle. For what it's worth, at the other location (in the garage) for the three way on the right there does appear to be a neutral bundle.   Also, there's a ground wire that doesn't go anywhere from that far right switch. Should I pigtail it into the ground wire bundle in the box that is in the other circuit?   Thanks for any help you can provide!