This is a multi-part message in MIME format. ------=_NextPart_000_015A_01C23301.8BD0DC60 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit > So what's a more efficient way to power the solenoid from the 5V supply? An excellent solution, and probably as cheap as you are going to get with a standalone solution and still save power, is the relay driver proposed on this list about a year ago OR Roman Black's 2 transistor regulator that was derived from that. See archives No - I'll attach it as it's very good From Richard Prosser 27 August 2001 re [EE]: re Design Challenge - Low power step down switching regulator. See long and "interesting" thread in archives if interested where it went from there. Russell McMahon _______________ How about something like this. The circuit attached is used to drive a 5V(~15mA) relay off a supply ranging from about 20 to 70V. I can't guarantee all the component values but they are about right. The coil is the relay coil and the current is sensed by R7. If R7 became the load and D1 became a zener at about 5.6V then it should work as a buck converter. Transistor types are BF422/BF423 or BSR19A/BSR20A for the smt version. Similarly the IN4148s are actually BAX12 or BAV99. D2 stabilises the feedback so that the hysterisis is stable with supply voltage. Input voltage is limited mainly by transistor & diode ratings. Richard P (See attached file: relay_~1.gif) ---------------------------------------------------------------------------- ---- ------=_NextPart_000_015A_01C23301.8BD0DC60 Content-Type: image/gif; name="relaydriver.gif" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="relaydriver.gif" R0lGODdhgASEA4AAAAAAAP///ywAAAAAgASEAwAC/4yPqcvtD6OctNqLs968+w+G4kiW5omm6sq2 7gvH8kzX9o3n+s73/g8MCofEovGITCqXzKbzCY1Kp9Sq9YrNarfcrvcLDovH5LL5jE6r1+y2+w2P y+f0uv2Oz+v3/L7/DxgoOEhYaHiImKi4yNjo+AgZKTlJWWl5iZmpucnZ6fkJGio6SlpqeoqaqrrK 2ur6ChsrO0tba3uLm6u7y9vr+wscLDxMXGx8jJysvMzc7PwMHS09TV1tfY2drb3N3e39DR4uPk5e bn6Onq6+zt7u/g4fLz9PX29/j5+vv8/f7/8PMKDAgQQLGjyIMKHChQwbOnwIMaLEiRQrWryIMaPG jf8cO3r8CDKkyJEkS5o8iTKlypUsW7p8CTOmzJk0a9q8iTOnzp0mAADg45On0KFcfAbNY/Qn0aVM oSQ9auep0qZUW0q9ijWr1q1cu3r9Cjas2LFky5o9izat2rVJq7qlNDVLW6RGN8V9i5fRXSxQ6XLa mzewIcBXCNcxPAmx4MV+FE9xHAfyI8mMK0f1QtlNZr2WOwPa3AT0GtGJSHs+jcZ0EtVnWBdyjTo2 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