Up/Down Timer For A Power Antenna

This up/down timer was
designed to control a power antenna on a late-model vehicle. Normally,
this vehicle uses a body computer to control the antenna. However, the
person who owned the vehicle wanted to install his own high-powered
audio stereo system. The original stereo system was tied in with the
body computer and this meant that a separate antenna controller was
required for the after-market sound system. Also, the power antenna
fitted did not have limit switches inside, hence the need for a timed
control circuit. Here’s how the circuit works. first, assume that the
radio antenna control output is not switched on – ie, the radio is
switched off.

In that case, relay RLYC will be off and so relay RLYA
will also be off, as is the motor. Conversely, when the radio is
switched on, the radio antenna control output line switches to +12V. And
when that happens, RLYC closes its contacts
and applies power to the circuit. As a result, C2 (330OF) quickly
charges via D4, while Q4 is biased on via D5 and R5. This ensures that
Q3 and relay RLYB remain off. At the same time, Q2 is is turned on, thus turning on RLYA and applying power to the motor. This drives the antenna in the up direction. During this time, C1 charges via R2.

Circuit diagram:

When the voltage across the capacitor reaches +8.1V, Q1 turns on via
ZD1 and so Q2 turns off and switches off the relay – ie, this gives the
“up” timeout. Using the values shown for C1, R2 and ZD1 gives an “up”
duration of approximately 6 seconds – long enough to fully extend the
antenna. D1 discharges C1 (via resistor R1) when the +12V supply is
later removed. When the radio is switched off (or a CD placed into the
stereo unit), the radio antenna control output switches back to 0V. This
does several things: first, it turns Q4 off and this allows Q3 to turn
on due to the stored charge in C2. Q3 and RLYB
now turn on for about six seconds – ie, while C2 discharges via R4 –
and this switches power to the motor in the opposite direction to drive
the antenna down. Diodes D4 and D5 are there to prevent C2 from
discharging back via the circuitry around on Q1 and Q2.

Author: Peter Howarth – Copyright: Silicon Chip Electronics

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