Lowe's has DeWALT Auto Electronic Air Compressor Tank Drain (‎DXCM072-0030) on sale for $55.87. Shipping is free.

Product Details:

• Eliminate manual draining
• Automatically drains condensation from the air compressor tank
• Designed for use with all air compressors
• Easy to install and maintain
• Set the drain to activate anytime between 30 seconds and 45-minutes
• Includes:
  • Electronic auto tank drain with 6 ft power cord
  • Clean out strainer valve,
  • 10 braided stainless steel tank drain tubes with 1/4 NPT elbow
  • 3 ft drain hose with hose clamp
  • Universal multi-purpose mounting bracket
• Maximum pressure: 230 psi
• Supply voltage: 120-Volt
• Connection: 1/4 in NPT