5
\$\begingroup\$

I'm building a battery operated robotic arm and would like to use a cheap, short distance RF transmitter/receiver kit to trigger it. However I would like to know whether the receiver will constantly be consuming power as it listens for the signal. I would prefer to not have to change the batteries more than once or twice a year considering the motor will only be running for a couple seconds per day.

\$\endgroup\$
  • 2
    \$\begingroup\$ What is the specific receiver (or circuit)? Some kits/packages such as the XBee have a timed "sleep" mode, where it consumes almost no power (obviously, it does not receive during this period). \$\endgroup\$ – Ron J. Jun 26 '13 at 2:51
2
\$\begingroup\$

As long as the receiver is turned on, it will consume power.

There are methods to reduce the amount of time it is turned on, however. For example, you can turn it on for 5 seconds every minute. On for 5 seconds, off for 55 seconds. Then the transmitter and receiver need to have synchronized clocks so the transmitter knows when to transmit. The exact timing of this depends greatly on your radio. You might have it on for 50 mS, and off for 950 mS.

You didn't say anything about what your system is, so I cannot say what your battery life will be or if 6-12 months is reasonable. My gut instinct is that you won't come anywhere close to that. I would recommend using an alternative source of power. Maybe solar or some form of energy harvesting. If your device is normally very low power then something like that can power the radio and MCU, saving the battery for when the motor is on.

\$\endgroup\$
2
\$\begingroup\$

Most receivers will consume power, when listening. (The only exception that I know of are RFID-type receivers.)

At the moment of writing, the specific receiver is not mentioned in the O.P. Here are numbers for a Bluetooth Low Energy module (BLE113).

  • Transmit: 14.3 mA or 18.2 mA
  • Receive: 14.3 mA
  • Sleep mode: 0.4 uA (micro-Amps)

Cyclic sleep with wake-on-radio (WoR) is a common way of reducing the average power consumption and increasing the battery life. This, however introduces latency. If the radio is receiving for 0.1 sec every minute and not receiving for 59.9 sec, then the worst case latency will be ≈ 1 min. As receive duty cycle decreases, the ampere-hours consumed in sleep mode will outweigh that during the active mode.

P.S.

The type and size of the battery is another major factor. Achieving a 6 months battery life with a 10 mm coin cell would be harder than with a pack of alkaline AA cells.

Internal leakage in the battery itself (self-discharge) will also influence battery life.

\$\endgroup\$
2
\$\begingroup\$

I've done a system where the radio woke up once per second and listened for 25msec and shut down if there was nothing sensible to detect. This means the duty cycle of the receiver was on for one-fortieth of the time thus current consumption went down (on average) from about 15mA to about 400uA. This could be made much leaner of course and the receiver could be shut down for (say) 10 seconds reducing the current consumed to below 50uA.

However, if you don't want the hassle of synchronizing transmitter and receiver, your transmitter has to repeatedly send the same message out for up to 10 seconds continuously so that it catches at least one 25msec time-slot when the receiver is powered and listening. I guess it all boils down to finding a receiver that can be up and running and listening in the smallest time period possible. The downside is that the shorter the time period it is listening for the less able it is to detect a bona fide transmission from the intended transmitter.

I don't think these are big problems unless you need "instant" response times from the motor.

\$\endgroup\$
  • \$\begingroup\$ One second latency is really good. Could you please share details about the system? \$\endgroup\$ – hulkingtickets Apr 18 '14 at 11:14
  • 1
    \$\begingroup\$ @hulkingtickets I used a PIC micro and a simple 418 MHz FM radio receiver module. The PIC was asleep most of the time and every second it woke up, activated the receiver (which could be up and ready in a couple of milliseconds). The output from the radio was analysed to see if there was a transmission preamble - this has to be sent for at least 1+ seconds to "align" with the receiver waking up. If the PIC detected a tx preamble it stayed listening till the message was received. \$\endgroup\$ – Andy aka Apr 18 '14 at 11:20
  • \$\begingroup\$ That's cool.How was the range, reliability ? \$\endgroup\$ – hulkingtickets Apr 18 '14 at 11:42
  • 1
    \$\begingroup\$ @hulkingtickets it depends on the transmitter power and if you up-vote LOL \$\endgroup\$ – Andy aka Apr 18 '14 at 12:01
  • \$\begingroup\$ 418MHz ain't bad in and around buildings but transmitter used was only qtr of a milli watt. Prob got over 100m line of site and 30m in buildings but there will be blindspots as in anyradio system. What are you looking for @hulkingtickets. \$\endgroup\$ – Andy aka Apr 18 '14 at 16:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.