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For a DIY project I would like to adapt a remote (RF) control

remote control case

to accept digital signals from MCU. I am not quite sure how the buttons in question work: Are they normal buttons which make near 0 ohm connection or something advanced (it's part of the question).

If I will be able to tap into the button terminals, which circuit would be suitable to "emulate" pressing the button? Can open collector / open drain on from the MCU side work (using common ground with remote control circuit) or additional components are needed? Maybe there are ICs to provide open collectors for 8 connections in at once (would be interesting to know how those might be called)?

For example, http://www.penguintutor.com/electronics/simple-homeautomation-raspberrypi uses relays, which I do not like.

(I can see similar questions like How to "push buttons" on voltage ladder? , but the answer does not contain more detail on transistor approach)

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Roman Susi
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  • A closeup of the button areas on the PCB would help to identify the used type. I guess they are just standard rubber/silicone pads with a conductive coating at the bottom. They will close the contacts on the PCB with around 10-100 Ohms resistance (black bottom) or ~1 Ohm or less (golden coating). – Arsenal Aug 19 '15 at 15:16
  • Yes, black conductive coating is there. When measured with voltmeter leads, shows 100 ohm., so I guess your estimate is right. (I will add photo if will get a good one) – Roman Susi Aug 19 '15 at 16:42

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Usually the keys are multiplexed so you cannot easily use an output directly.

You could, however, emulate the key closures with an analog switch such as a 74HC4066 quad analog switch. The ground on the remote should be commoned with the controlling circuit. Power for the 4066 should be the same as the MCU I/O power.

You would send a pulse to the analog switch to emulate the key press- maybe 250ms.

Spehro Pefhany
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  • 4066 -- nice substitute for a relay in this case. – tcrosley Aug 19 '15 at 16:40
  • Surprisingly, can't find any information on how the multiplexing is usually done except for some patents (which may be not the case). Any references? Or is it so I can pretend 4066 abstracts SPST switch well enough for possible multiplexing? – Roman Susi Aug 19 '15 at 16:46
  • Here is an example, but yes as long as the grounds are common and your power supply voltage (looks like 2AA cells, so 3V) is not much greater than your microcontroller supply it will behave like a switch. You just have to control it and ignore the signals the micro is sending through to scan the keypad. – Spehro Pefhany Aug 19 '15 at 17:36
  • According to datasheet (NXP one) for 74HC4066 it's ON resistance is about 80 ohm at 2 V (I guess, slightly less for 3V), polarity dependent. So, I need to check if it is enough to "press" the button. I have HEF4066B, but that one has even higher ON resistance. – Roman Susi Aug 19 '15 at 18:06
  • Yes. 180 ohm and even 1k triggered the circuit. Will wait for 4066 parts to implement. Thanks! – Roman Susi Aug 19 '15 at 18:15
  • The black carbon keyswitches are usually in the 100 ohm range, and usually they'd design it to work with several hundred ohms at least. The HC4066 is 100 ohms max at room temperature and 4.5V supply so just the same. The supply voltage will be the MCU supply, not the 3V supply in the remote, so the resistance will be very low. – Spehro Pefhany Aug 19 '15 at 18:18
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    I've checked with one on-off pair (manually, without MCU) and the design seems to work. Pull-down resistor on 4066'a control pin was required because otherwise switch activates by itself. – Roman Susi Aug 21 '15 at 18:44
  • Pull down/pull up will not be required when it's connected to the MCU, but you should tie unused inputs to GND or Vcc for the 4066/MCU. Glad it works for you. – Spehro Pefhany Aug 21 '15 at 18:46
  • Connected to Raspberry Pi. Pulldown (15k) still required. Maybe, it can be configured via software, but it I think it is better to have to prevent random firings during boot and such. – Roman Susi Aug 22 '15 at 09:13
  • @SpehroPefhany I have encountered one subtle problem: when remote and controller share GND, powering off controller causes some buttons to be "pressed" and remain in that state all the time. Disconnecting remote's GND helps. All 4066 inputs are pulled down, same for 3-8 decoder (74HC238) inputs I am using in front of 4066 – Roman Susi Sep 07 '15 at 18:23
  • (I meant 74HCT238 in the previous comment) – Roman Susi Sep 07 '15 at 18:29