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Chip Help > Hardware > Components

Wondering about substituting a part, finding a part, or just looking for more information? Here are some of the questions that we have received:

What are SOICs and PDIPs?
SOICs and PDIPs are the sizes of chips we offer. For more information and visual comparisons, please visit our About ELM page.


What material do you use for your chip labels?
It's a high-temperature polyimide material that is specially produced for this purpose (W.H.Brady type B-652). It can be exposed to as much as 300C/572F for 5 minutes (wave soldering), and 176C/350F continuously without damage.


Is the label adhesive permanent?
Yes. When initially applied, they can be repositioned, but after a few days become permanently affixed.


Do I have to use the 2N3904 and 2N3906 transistors?
No, pretty well any general purpose transistor should work. For the 2N3904, there are many types that can be used - look at the BC546, BC547, BC548, BC549, BC550, 2N2218, or the 2N2222. Similarly, there are many options if you can not locate a 2N3906 - look at the BC556, BC557, BC558, BC559, BC560, and the 2N2905 for example. As long as the transistor has reasonable gain, it should work as the required frequency response is not too high.


I can't find a 78L05 regulator. Will a 7805 do?
It will work, as the output voltage is the same, but you should understand that the two regulators limit the output current to different values. The 7805 will allow more than 1A of current to flow if the output is shorted, and this could damage components or wiring. For some of the OBD circuits, the 78L05 was specified in order to limit currents to a safe level. You can substitute, but be aware of what you are doing.


I do not live in North America, so I substituted a PAL crystal for the NTSC one shown in your ELM624 circuit but am still having trouble. What next?
Put the NTSC crystal back in the circuit! That frequency was chosen because those crystals are inexpensive, and readily available. It has nothing to do with the video parameters. By using a different frequency, you have shifted all of the timing proportionally.


Can I use a 3.6864Mhz crystal instead of the 3.579545?
No. The circuit's time delays are all calculated based on the use of a TV type colourburst crystal being connected to the chip. While the oscillator will still function if another frequency crystal is used, the period of the oscillator will be different, and all timing will be in error. You can certainly try the other frequency, but don't be surprised if the circuit doesn't work properly.


Do I have to use 27pF capacitors with the crystal?
No. The loading capacitors depend on the crystal chosen, so some experimentation may be required. As a starting point, the 27pF or possibly 33pF should be a good value.


My crystal says to use 15pF of loading capacitance, but I can't find 7.5pF capacitors. What should I do?
Use the 27pF shown in the example applications, and failing that, use 33pF. Many people initially think that the two capacitors are essentially 'hung in parallel' across the circuit, so automatically assume they need half the value for each. You have to put yourself in the crystal's position however, to see that the two capacitors are in fact in series across the crystal. The 27pF shown, along with typically 5pF or more of stray capacitnce across them results in about 16pF of effective parallel capacitance.
Having said this, in a pinch anything from 22pF to 47pF will likely work just fine with almost any crystal.


Can I use a MAX232 IC for the RS232 interface?
Yes, it can be used but you need to be aware that the RS232 Rx inputs on the ELM320, 322, 323, 624, etc. are all inverted from what they should be (for a standard interface). By placing another inverter in series with the Rx input, Maxim's interface IC's can be used.

Here is how you might typically connect a MAX232 to provide both the RS232 interface, and the inversion on the Rx input:

max232

Note that the ELM327 does not need to have its Rx input inverted. To use a MAX232 with the ELM327, simply connect it as 'normal' - Rx would go to pin 9 in the above circuit, and the inverter between pins 12 and 13 would not be used.


Where do I get a male J1962 connector?
This is difficult as they are not that easily found. Here are some links that may help:

OBD2AllInOne carries components for you to assemble your own, as does OBD2Cables.

Complete cables are available from OBD2Cables, ScanTool.net, B&B Electronics, and Multiplex Engineering (but there is a minimum order).

There are likely several others. Why not Search the web for other vendors?


Where can I buy a circuit board for my ELM IC?
Elm Electronics does not sell any circuit boards, but several vendors offer boards and kits. You may find one from the links on our OBD links page, or possibly prefer to make your own.

If making your own pcb, we can help get you started for the ELM327 only. We've posted some of the design information for our (3"x4" single-sided) demonstration board that was used for beta testing:

Demo Printed Circuit Board Picture
ELM327 Demonstration Board

Here are the supporting files for you to download:

Mac Users can 'option-click' the links to download, while PC users can 'right-click' and save the target.

Note: You are free to use these as you wish, but please note that Elm Electronics Inc. makes no claims, and offers no guarantees as to the circuits' performance.



Do you have any information showing typical voltages for the above circuit?
Yes - have a look at the AN02 - ELM327 Circuit Construction Application Note.



I can not find a 0.3" wide 28 pin socket. Can you help?
Why not use two 14 pin sockets, end-to-end? Many brands can be connected this way, will work fine, and are likely a lot less expensive than a 28 pin one.


My OBD cable uses pin 2 for common, but I see that your demo board uses pin 1. Is that OK?
Yes - the demonstration board was only created for testing the IC, and was not meant to cover all situations. We used cables from OBD2Cables.com for our testing, but you may obtain your cables from anywhere (and modify the board connections to suit). Perhaps you may wish to use both pins 1 and 2 of the connector for your circuit common.


Your other OBD ICs use a 3.58 MHz crystal. Can I use that for the ELM327?
No - and do not use a 4.096 MHz crystal either - the frequency is too far off. You must use a 4.000 MHz crystal, as shown in the data sheet.


Do I have to use a Microchip MCP2551 transceiver chip with the ELM327?
No - a CAN transceiver is a very common product that most of the major IC manufacturers make - you can choose as you like. To get you started, have a look at the 82C251 from Philips, the SN65LBC031 from Texas Instruments, and the LT1796 from Linear Technology. We only show the Microchip MCP2551 as one possible example.



Chip Help > Hardware > Components
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