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Frequently Asked Questions

Here are answers to some of the common questions visitors to Electronics 2000 ask. Please read this page before contacting me as you may find the answer here. If it isn't or you need more help click here to contact me. I'll try to answer you as quickly as possible, but unfortunately I don't always get time to respond to all messages. You could also try the forum in case other visitors can help you. More questions and answers will be added as they are asked.

Q: I've seen the connector I need on your web site but can't see a price, nor how to order it. Where is the link to place an order?

A: Sorry, Electronics 2000 only provides information and does not sell any components or connectors. Try a supplier in our links section.

Q: I want to put a USB interface on my project but don't know where to start. Can you help?

A: Take a look at www.ftdichip.com. They have some very good USB interface ICs and supply free drivers. Altenatively Microchip have a range of PIC microcontrollers with USB built in (www.microchip.com/usb). They supply sample code for a variety of device classes, however these need more experience of USB and PIC programming to use than the FTDI devices.

Q: I have an AC power supply connected to a PCB via a plug, where it is rectified and regulated. I fitted a small ferrite to the power supply cable to help prevent interference, as well as a varistor on the PCB to absorb surges /spikes. However the ferrite is not suitable due to lack of space. Is there anything else I can put on the power connector or the PCB to prevent interference.

A: Ferrite cores around cables are normally used to prevent EMC emissions more than to prevent interference entering equipment. Normally a reasonably large electrolytic capacitor on the power supply input with a small (0.1uF) ceramic capacitor in parallel with it will suppress power supply noise. If you still have problems you could also try a 3-terminal capacitor - a capacitor with built in ferrite beads on the legs. These are available from RS and other sources. In critical applications the design of the PCB can also affect interference. If the circuit is all digital (e.g. a microcontroller with no analogue inputs) then interference is unlikely to be a big problem. If you have an oscilloscope you can easily check for noise problems.

Q: I am trying to run between 1-6 LEDs in parallel rated at 3V 20mA each off of a power supply rated at 3V 500mA. I keep burning out some of the LEDs when I do this. I realize that I need a resistor to limit the current but your calculator does not have a place to factor in the supply current rating. Could you please tell me what resistors I would need to solve this problem. I would like to use just one resistor right after the power supply before it enters the LED's rather than one resistor for each LED, if possible.

A: Firstly the 500mA rating of the power supply is the maximum current it can supply, and plays no part in the resistor calculations. You just need to check that the total current drawn by the LEDs is less than this. If you want to connect them in parallel you must have 1 resistor in series with each LED, otherwise more current may flow through one LED than others and overheat it.
If you use the calculator you will get a resistor value of 0 ohms, as the supply voltage is the same as the LED forward voltage. In practice I suspect your power supply is unregulated and is giving more than 3V. You should measure this with a multimeter and calculate the resistors based on this, e.g. if the supply is actually 4V then the resistor should be (4-3) / 0.02 = 50 ohms. You can use the online calculator to work this out (enter Vf as 3V and If as 20mA and the supply voltage to get the resistor value). In fact 3V is a bit high for most LEDs, standard LEDs have a forward voltage of around 2V.
A better option would be to use a higher voltage supply with the LEDs in a series / parallel combination. For each parallel branch of LEDs in series 1 resistor is needed. This can be calculated using the formula (supply voltage - (forward voltage * number of LEDs in series) / forward current. Ensure the supply voltage is higher than the voltage dropped by the LEDs. More LEDs can be added by having more parallel branches.

Q: I am having trouble finding the right resistors that I need for my application. I have five ultra bright blue LED’s and an 18 volt transformer. Each LED is 3.3V at 20mA. What value of resistor do I need? I am not sure what OHM means and the also not sure what power rating and tolerances I'll need.

A: Assuming the transformer you are using is actually a power supply unit (i.e. giving 18V DC not 18V AC) the resistance needed in series with all 5 LEDs is ((3.3*5)/0.02) = 825 Ohms. Ohms is the unit resistance is measured in. The nearest value you can get to this is 820 ohms, although 1K (1000) ohms would also be OK. Connect all 5 LEDs in series along with the resistor. The power dissipated in the resistor would be ((0.02^2)*825) = 0.33 watts. Hence you will need a 1/2 watt resistor. If the transformer output is AC the resistance needed will be different.

Q: If a calculation result says I need a 120 Ohm resistor and I can only get a 100 Ohm resistor, how does that affect my circuit? Or, if I calculate 150 Ohm and have a choice between 100 and 220, which do I choose?

A: In general round values up to the nearest higher preferred value, but it depends on the difference between actual and available values. For applications such as current limiting resistors it is best to round up rather than risk overloading something. If a resistor only slightly smaller than that calculated is available this is OK. If the resistor is quite high (in the order of kilohms) then it is not so critical. Bear in mind also that unless you are using resistors with a high tolerance the actual value may vary considerably anyway.

Q: I'm trying to install a mobile phone hands free unit to my car and need to identify the mute connection on the radio - I believe the pinout is standard on European cars. Do you have a diagram? The radio has two sockets - 1 for the power and speaker connections and the 2nd for a remote control.

A: The connector will probably be the ISO standard type. This is actually 2 connectors, 1 for power and 1 for speakers, but the 2 are often joined together. See http://www.bluespot.co.uk/stock/iso.asp for ISO connector wiring. It shows the plugs separate, you can tell which is which as the speaker one should have all 8 leads, all the same thickness. The power one will probably have less than 8 wires, and at least some will be much thicker. It might also be marked on the radio. The phone mute is on the power connector as in the diagram. The remote control connector might be standard, but is probably specific to the radio model or manufacturer.

Q: I'm having trouble identifying some components in a small kit. I am unable to identify the capacitors and have so far failed to correspond certain parts with their respective slots.

A: Presumably your kit didn't come with instructions on how to identify the components (not very helpful). Most capacitors are identified by a 3-digit code. See here for details of how to decode them. Those without the 3-digit code are harder - having decoded the others you may be able to identify these through process of elimination. If there is a component layout printed on the circuit board this may help, and some capacitors may also only fit in certain holes. Alternatively some multimeters are able to measure capacitance. If all else fails try contacting the kit manufacturers.

Q: Can you explain how a potentiometer works and how it can be used as a volume control?

A: A potentiometer is basically a variable potential divider. Information on the operation of a potential divider is in the technical data section. Looking at the diagram, a potential divider has 3 connections, one at the top, one in the middle between the two resistors and one at the bottom. These correspond to the 3 connections on a potentiometer, the middle one being the wiper. Think of the potentiometer as a potential divider where the track forms the two resistors with the join between the two (the middle connection) being the wiper. As you turn the potentiometer this point moves from one end of the track to the other.

When used as a volume control the input signal is connected to the top of the divider (one end of the track) and ground connected to the bottom of the divider (the other end of the track). The output is taken between the middle of the potential divider (wiper) and ground. With the control at full volume the wiper is at the top of the track so the top resistor is very small and the bottom one very large. In the middle both resistors are the same so half the input is tapped off to the output. At minimum volume the wiper is at the bottom of the track so the top resistor is very large an the bottom one very small. Hence no signal goes to the wiper. The value specified on the potentiometer is the resistance of track measured between the track end terminals. Summing the resistance measured between each track terminal and the wiper in turn will give the same value.

Finally, several potentiometer types are available, the most common being linear and logarithmic. The latter is used for commonly volume control; it has a logarithmic track to compensate for the way human hearing works.

Q: I have tried to download Electronic Assistant, but another piece of software was installed instead, can you please tell me what to do?

A: This is a known problem with Electronics Assistant version 3.3, and is related to the self-extracting distribution file, not to Electronics Assistant itself. The problem does not occur with version 4 and above.

Q: How much does it cost to register the Electronics Assistant software - the site I downloaded it from did not say?

A: Electronics Assistant is currently freeware so there is no charge. If you downloaded it from anywhere other than this site please let me know so I can check it is not being advertised as shareware. If anyone wants to distribute Electronics Assistant via any method please contact me to let me know. All I ask is that you distribute the package in its entirety and that you provide a link to this site.

Q: I would like to use your interactive formulae calculator scripts on my own web site. Would you grant permission for this?

A: Depending on the nature of the site this may be possible subject to certain conditions being met. Please contact me with details and a link to the site you want to use the scripts on, or a sample page if the site is not yet online.

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