Line-Level Controlled Relay-0.5 to 5 s Delay

Ideal Use Cases

The RDL ST-ACR1 is designed for systems in which a line-level audio signal must initiate relay action. Its adjustable trigger range lets the module respond to signals from -30 dBu through 0 dBu, while the audio-threshold LED provides a direct reference when setting sensitivity. It is suited to audio-presence control, automatic switching tied to program level, and installations that need a relay state to follow whether audio is above or below a selected threshold. The 10 kΩ balanced bridging input may be connected balanced or unbalanced, supporting either connection method specified for the module.

Why Choose This Model

The ST-ACR1 combines adjustable sensitivity, an adjustable relay-release delay, and double-pole-double-throw contacts in a compact enclosure. The SENS potentiometer determines the audio level required to trigger the module, and the threshold LED helps the installer make an accurate adjustment. The DELAY control determines how long the relay takes to release after the audio falls below the selected threshold. Its nominal adjustment range of 0.5 to 5 seconds allows the release behavior to be matched to the source signal. The module measures only 0.7 inches high, 3 inches wide, and 1.6 inches deep.

Installation and Integration

The audio input is a 10 kΩ balanced bridging connection and can be wired for balanced or unbalanced operation. Power must be ground-referenced 24 Vdc at 50 mA; the stated requirement assumes a maximum 20 mA load at the SLAVE output. Recommended power supplies, which are not included, are the PS-24AS for the USA and PS-24AX for international use.

In addition to its DPDT relay contacts, the ST-ACR1 provides an open-collector control output rated at 25 mA. This output is suitable for driving indicators or a slave LCR relay. When selecting a switched load, the installation must remain within the published limits of 2 A maximum current, 220 Vdc or 250 Vac maximum voltage, and 60 W or 125 VA maximum switching power.

Performance Highlights

• Adjustable audio-input sensitivity from -30 dBu to 0 dBu.
• Nominal relay-release delay adjustable from 0.5 to 5 seconds.
• Activation response time of 25 ms.
• Retrigger delay of 250 ms.
• Double-pole-double-throw relay contacts.
• Audio-threshold LED for setting input sensitivity.
• Maximum switching current of 2 A.
• Ambient operating range from 0 degrees C to 55 degrees C.

The fast 25 ms activation response allows the relay to react shortly after the input crosses the selected threshold. When audio falls below that threshold, the adjustable release delay controls when the relay returns to its released state. The 250 ms retrigger delay is also specified as part of the module’s operating behavior.

What Makes It Different

The defining feature of the ST-ACR1 is its combination of audio-level sensing and directly adjustable relay timing. Sensitivity and release behavior can both be configured on the module, while the threshold LED gives visual feedback during setup. This provides more control than a fixed-threshold, fixed-delay arrangement without requiring the source to provide a separate control signal. The DPDT contacts provide two switched poles, and the additional open-collector output can operate an indicator or compatible slave LCR relay. These functions are packaged in a module weighing 0.15 pounds by WEEE weight, with a listed shipping weight of 0.2 pounds.

Final Assessment

The RDL ST-ACR1 is a compact, purpose-built solution for switching based on line-level audio. Its -30 dBu to 0 dBu sensitivity range, threshold indicator, 0.5-to-5-second release adjustment, 25 ms activation response, and DPDT contacts provide clearly defined control over triggering and release. Balanced or unbalanced input compatibility and the 25 mA open-collector output broaden its integration options, while the published current, voltage, power, and temperature ratings provide the essential limits needed when planning an installation.