Radio systems & failsafe

The link

Combat robots use off-the-shelf 2.4 GHz hobby R/C:

• Transmitter (TX): an “air” radio (gamepad or tray/stick, for drones and planes) or a “surface” radio (pistol grip, for cars). Air radios are far more common. Buy once — a good multi-model radio lasts across many robots.
• Receiver (RX): bound to the TX, outputs servo (PWM) or serial (S.BUS / CRSF) to the ESCs.
• Protocol: the receiver has to match one the transmitter speaks.

Current picks¹

System	Protocol	Notes
RadioMaster Pocket / Zorro (air) + ER4/ER3C-i RX	ExpressLRS (ELRS)	The current recommendation for a first radio — best performance, open-source, EdgeTX, cheap. Only slightly more than a FlySky.
RadioMaster MT12 (surface)	ELRS or 4-in-1	Best surface option; same customisability.
FlySky FS-i6 / i6X + FS2A RX	AFHDS-2A	The long-time budget default; still fine and very common, and what many combat “motherboard” boards with built-in receivers expect.

Older or cheap gamepad radios can have poor range or failsafe behaviour — check community reports before buying for combat.

Standard channel map (air radio)¹

• CH1 / CH2 — left / right drive (mixed into arcade control, below).
• CH3 — spinning weapon (throttle stick — no self-centring spring).
• CH4 — bidirectional arm (lifter, hammer, flipper) on the rudder stick.

Setup essentials

• Binding: pair RX to TX; re-check the bind after firmware updates. ELRS uses a bind phrase set over the TX/RX WiFi hotspot — the same phrase on any receiver pairs it.
• Model memory: one named profile per robot. Prevents driving robot B with robot A’s mixes.
• Drive mixing: two mixes turn independent CH1/CH2 into arcade control — a +100% mix (CH1 → CH2) for forward/back, and a −100% mix (CH2 → CH1) for turning. Then reverse motors / swap channels until forward is forward and turns go the right way. Many combat dual ESCs mix onboard instead — then send raw throttle and steering. See the Bristol Bot Builders mixing guide.²
• Endpoints: span the full −100% to +100% so the ESC sees its whole range.
• Invert switch: flips drive (and sometimes steering) for inverted running.
• Weapon on a switch or the throttle stick, ideally with an arming step so it can’t spin up from a stick bump.
• Expo: most builders set some negative expo on steering — calmer around centre, full rate at the ends.

Failsafe — the critical part

On loss of signal, every output must go to a safe state:

• Drive → neutral / stop (1500 on CH1, CH2, CH4).
• Weapon → stop — zero throttle, which is 1000 on CH3, not the 1500 centre and not “hold last”.
• Where to set it depends on the system: on the receiver for ELRS and FlySky (and note the two FS2A chip revisions behave differently — the “rectangle chip” learns failsafe from a button press, not TX values). Confirm ESC behaviour on lost signal too. “No pulses” works with some ESCs but not all.¹³

Verifying failsafe

1. Robot on a stand, wheels clear, weapon removed or in a test box.
2. Power up, bind, arm, apply some drive and weapon throttle.
3. Switch the transmitter off.
4. Drive must stop and weapon must stop within a fraction of a second.
5. Repeat after any config change.

This exact test is performed at tech inspection.³

References

Footnotes

1. Ping, L., with L. Skotiniotis, I. Talbert & D. Tran. Beetleweights for Dummies (Fall 2025 revision) — builder-maintained beginner guide distributed via the NHRL Discord. Sections: Control Systems (Transmitters and Protocols, Drive Mixing, Setting Failsafe). ↩ ↩² ↩³

2. Bristol Bot Builders — binding, mixing, and failsafe guides, plus the tip-speed / drive-RPM calculator. ↩

3. SPARC Robot Construction Specifications, v1.4, 2023 — radio failsafe requirements and the tech-inspection failsafe test. NHRL documents the same test for its events. ↩ ↩²